Things Special Editions should contain:
Artbooks/Storybooks (i.e. like the one in Mass Effect),
OSTs,
Decent figurines.
T-Shirts (though annoying as they are usually one size only! (usually Large...)),
Bonus DVDs with making-of and behind the scenes videos are nice, (but not entirelly necessary!).
Things that special editions have which are pretty useless:
Minor DLC, (i.e more costumes, an extra gun ect.),
Stickers, (Ok if they come with some art books/printouts, but pointless on their own!)
Crappy USB datasticks, (like 1gb or 2gb, you can get 16gb ones now for a good price so why bother!),
Cheap and fragile figurines that break easily.
Games that do it right: Fallout 3, Bioshock 2, Blazblue, Halo 3 (don't own these but I would want them, if they were cheap enough! :D)
Games that have reasonalby decent special editions: Mass Effect, Mass Effect 2 (Don't like DLC and lack of OSTs in these games!),Street Fighter 4
Games that don't do it all that well: Forza Motorsport 3 (off the top of my head, but all it had was DLC and a datastick really...)

I got the Bioshock one with the Big Daddy figure dirt cheap because the figure was broken. Fortunately it was a clean break and one of the guys at my local modelling club that I go to fixed it with a pin and decent glue. Good as new.
The Saints Row 2 special edition had a crappy gold gun style box and a bullet shaped USB datastick of only 1GB
http://www.consolemonster.com/newspost.php?id=0000003948
The Ezio figure in the Assassins Creed 2 special is pretty good quality. Its a sturdy plastic.

The cheaper I can get the game, the better. I generally don't care about extras; I almost never even watch the special features on DVDs, actually. Even my desire to have something physical on my shelf has mostly eroded (though I haven't fully embraced digital distribution, either, because there's often an additional layer of DRM).
There's also the fact that I almost never spend more than $20 on a single game, unless it's one that I want badly. By the time I get around to playing the big-name titles, their collector's editions are long gone anyway.

Art book?! I didn't get an art book... =(
That's really a kick in the pants, it's usually the reason why I get Special Editions.
As far as good S.Es go: Fallout 3 for the packaging and bobble head, GTAIV for the safe box and nicely bound art book (and OST, keyring, etc), Guild Wars: Whatever for the art books, mini pets and little extras, and The Witcher, art book and just being awesome.
Some of the early S.Es were pretty average. STALKER: SoC was pretty much a tin case with some exclusive download that wasn't appealing, FEAR just contained a small comic book, and I can't remember any more.

The worst "collector's edition" I've ever seen is the European Myst IV, the best thing about it is the thin carton "book" cover it comes in, total size being like two standard DVD cases.
In one half you have the game in its DVD case, and in the other you have a bonus DVD in a carton sleeve containing 5 videos, each being 3 to 4 minutes in length, and 9 songs from the soundtrack of 24 (they're even numbered: tracks 1 to 8, and 11). Not only is there a completely worthless DVD and a half-decent-looking box, there's also an INCOMPLETE playing card deck! 32 cards, not 52, cards 2 to 6 in each suite just isn't there.
Yes, I bought it, not knowing better, why do you ask?
Myst III and Uru were nearly as bad, but at least they had the whole soundtrack on audio-CD and, Prima guide book (Myst III) can come in handy for some of the puzzles, and the Prima "travel journal" (Uru) is a nice touch.
World of Warcraft did it right, with soundtracks and art books in both the original game and expansions so far, as well as a game map of the areas introduced in each part (the two expansions as mouse pads, the original as a rolled-up "cloth" sheet). Also in-game pets that has everyone that didn't buy the coll. ed. asking where you got them, especially the panda.
Ones that were in various places in between:
* Fallout 3 with its lunch box, bobble-head , and small art book was pretty good, though I miss the soundtrack.
* I liked the coin and "Pocket guide to the Empire" book that came with Oblivion, still no soundtrack though.
* Mass Effect 2... I don't care for the armor, but the comic, art book, and metal box were pretty nice. The extra DVD just contains over a dozen wallpapers and a making-of DVD video. I don't know much about the cerberus network as I have barely done more than start up the game a couple times.
* Dragon age: Origins: Metal box looks nice, cloth map does too, some lousy in-game items I couldn't care much less about, a dozen wallpapers. Oh, hey, a soundtrack of 18 tracks, just MP3 but that's better than nothing.
Most or all of them has that "making of" video in one form or another, as well as advert videos. This doesn't really add value by my counting, hence why I didn't bother mention mention it 'til now.

In this country, most are overpriced, cut down versions of what other countries get. Even Fallout 3 lacked the Pipboy alarm clock that the US got.
Not to be a suck up, but one of the best I've seen in terms of value for money was the Witcher Enhanced Edition (retail). With the short story, game guide, music CDs and video DVDs etc.
Mass Effect 2 also came with rather a lot and proved well worthwhile.
Dreamfall included its prequel and an art book. The prequel arguably being a better game.

Best Collectors Edition i've bought is the Alan Wake one, with Bioshock 2 closely after it. I wasn't too keen on Bioshock 2, but the collectors edition had some really nice stuff in it.

I was thinking of getting another PSP since I really miss mine, and was thinking of getting the Peace Walker bundle. I was ready to until I saw what it came with.
First off: The standard bundle's PSP is a metallic green. The thing looks like it belongs in a Christmas tree. You also get the game and a 2gb memory stick. Woooooo.
Then I notice a "Big Boss Bundle". It's the same price, and it comes with an even uglier PSP (some crappy camo pattern that looks horrible), a 4gb stick, a "premium" carrying case thats just a zip up sleeve, and a voucher.
So I decided to just get a plain one :P

Personally I think that the Rapture Edition looks like better value for money. Shame about the lack of music CD with that one though.

See that I wouldn't call a Collectors edition, it was just a CD-Rom and a Floppy disk version when there was the introduction of CD-Rom drives. That's not what I'm really getting that. Mind you, it was better to play Gabriel Knight with the voice cast rather than text. :)
Oh yes, I forgot about the Guild Wars one, I managed to get Factions for about a fiver for the collectors, so it was well worth it.
Shalgroth, heres the link to the collectors. From what you'll see its not a big artbook
http://www.play.com/Games/PC/4-/5483411/Fallout-3-Collector-Edition/Product.html

I got the mass effect 2 collectors edition because it was only $10 extra, they had copies left in the shop and I didn't have anything else to buy. It came with a making of DVD, a comic, an art book and some DLC codes, each of which I've used exactly once. I SUPPOSE I got $10 worth of entertainment out of them but it was, ironically, nothing special.
The Alan Wake special edition looks to be rather good though, a mate got it last week and its got a soundtrack CD (which is the kind of thing I'd use more than once) and a novella sort of thing that gives extra backstory which should enhance enjoyment of the game
Toy NVG's are still lame, no matter what way you look at it, especially given that you hardly ever used them in the game

Thanks for that. Heh, I just opened my lunchbox and there is an artbook.. I can't believe I didn't even notice it/remember it. The art books are a major reason why I bother with Special Editions.

Best one for me would be either MGS4, Alan Wake or Fallout 3. Can't decide. All have some awesome stuff.
Worse? I think Heavy Rain - the box looks nice... and that's it.
I don't have many CEs, so not much to choose from :)

The SC2 Collector's Edition looks awesome! :D