This is a question for the GOG team -- can we anticipate releases of Myst III, IV, and V? I played III back in the day and loved it, but never got a chance to finish the series. I will gladly pay you to help remedy that mistake!
Oh, and inclusion of the soundtracks would rock.

Myst III isnt so big, I think, but on install, Revelation (Myst 4) was 9 GB on TWO(!!) DVDs. I'm not sure about Myst 5, but I can tell you it was a BIG disappointment.

I liked Myst V. Sure it didn't live up to the first two games, but it was a hell of a lot better than the total irrelevance of III or the hammy new-agey style of IV. The puzzles were solid, too, with the slates being an interesting addition to the series.
Honestly, the only real disappointment for me was that it, being the last chapter of a franchise that had spanned 4 main games, an online entry and three novels, dealt almost exclusively with a new conflict unrelated to anything else in the series. Taken on its own I liked it, but it wrapped things up about as nicely as..... well, as the third book did.

myst 3 & 5 - 1 dvd & myst 4 - 2 dvd - this one was my favourite

I love Myst 4. It is my favorite of all of them, though Riven comes in a close second. The story of 3 was fitting and necessary.

I don't see where the story of 3 was necessary at all. It didn't do much to advance the plot of the series other than explain how not all the D'ni were gone and Atrus was building them a new age, but yet that really had nothing to do with the game...it was just the MacGuffin they used to tell Saveedro's story and lead you to ages where you solved a bunch of arbitrary self-contained puzzles.
Riven was so much harder than Exile because the puzzles were interconnected. You often couldn't fully solve one puzzle until you solved another one, and they rarely gave you a hint to let you know where you needed to go to find that answer.
Exile fits into the Universe, but it's not a story that desperately needed telling.
I know that 4 goes back into the story of atrus and his sons and what happened to them. I haven't played it yet, but I hope to find a copy of it soon. Playing Myst and Riven again from GOG really made me want to play the ones I never got around to.
I would be surprised to see IV and V on here, because they're still selling for 40 dollars if you can find them. Maybe III is old enough to make it onto GOG. but I doubt Ubi would let them list IV and V at even the 10 dollar price point.

Uru is probably the best of all the post-Riven games, but if I had to order them, I'd say V>III>IV.
None of them are really bad, but something about Revelation never quite sit right with me. I thought Achenar's redemption was an interesting touch, but I found the actual endgame somewhat lacking - rather like having Indiana Jones enter Atlantis only to find out it's a massive Candy Land board, complete with all the characters thereof; with two incredibly difficult RPG boss fights (on the order of SMT's Demi-Fiend and FFXII's Yiazmat) at the midpoint and end.
Exile was probably more thematically consistent with the rest of the Myst series (even if it felt like they picked up a dictionary and chose names at random - I mean, seriously, "Saavedro" and "Narayan"?), but it felt more contrived overall. It's got spectacle, sure, but it feels as if you're being led through what amounts to a theme park - and I don't just mean the Amateria age (which was, admittedly, awesome at the end).
As for End of Ages? It felt more like a proper continuation - even if it did pick up Uru's story and hence some of its incongruous mysticism. I thought that the best bit was at the end, finally concluding that Yeesha was not the Messiah figure she'd been played up as, and that in a nod to the original game, where you are given a choice of people to trust, you instead take the third, hidden option - to trust neither and find the truth hidden right in front of your eyes.

This'll be why I've only solved one puzzle in the four hours I've spent so far on Riven. I've got absolutely no idea what I'm supposed to be doing. I really liked Myst III, which was the first one I played. If it's $5.99 I'm sure I'll get it again. I hope it runs better than realMyst.

Quality aside, Myst 3 and 4 were made by Ubisoft, and Cyan's association with Ubisoft has apparently finally ended, so that may be an obstacle in any future publishing of those two games. Myst 5 and Uru were made by Cyan and published by Ubisoft, so they're in a much grayer area where it's conceivable that Cyan may have retained the rights, but 3 and 4 will probably need some agreement between Ubi and Cyan to get them published anywhere.

Well, we do have Ubisoft as a publisher on GOG, and now Cyan, so, perhaps, we can get III and IV. <_<
Riven was a tough game for me, but I kept at it, mostly because I just loved the imagery. It reminded me of an Infocom game, at least as far a world-building went. :)
V confused the heck out of me. I visited quite a few areas, but I didn't get very far in any of them... I think. Though it was nice seeing the Uru storyline continuing, and seeing areas that I assumed would have been visitable in Uru, had it continued. Also, of all the Riven parts that were crashed around the desert, the one iconic part I was looking for seemed to be missing, until I played End of Ages. :D
...
(enters volcano)
"Oh, there it is!" =^_^=\/

I really enjoyed III. Who cares if it was "out of canon".

I loved Myst III Exile. That said, it's the first and only Myst I played (for now at least) so I can't say much about coherence with previous episodes.

Myst III was pretty good in terms of puzzle design, but didn't think things through emotionally. The ending was the worst: the final puzzles are built around keeping the bad guy caged up so he doesn't ruin his chances of getting home by bashing your head in at the first opportunity, even after it's made perfectly clear that you can help him. And then once you successfully free him without letting him near you, he gives a thankful wave and heads off into the sunset to soaring music as if you're all-of-a-sudden supposed to have fuzzy feelings for this murderous idiot. Careless writing.
Myst IV was a lot better. Second best of the series for my money (behind Riven). I liked the "new-agey" stuff - it was surprising and even moving, and helped give the game its own personality, instead of just riding the coattails of its predecessors. It did stumble with a few puzzles though, and sadly the last one was one of the biggest letdowns.
But if you want excellent and insightful reviews of the Myst games (and almost every other first-person adventure game), there is no better resource than Andrew Plotkin's reviews at [url=]http://www.eblong.com/zarf/gamerev/index.html[/url].

So I'm looking to play the rest of the games. I've got IV, though it runs like CRAP on my Windows virtual machine...I think I need to find a straight-up mac version if they ever made one, but there's probably no guarantee it will run on snow-leopard since they ditched all the powerPC binaries.
What order should I play the last 3 games in? I've played the first 3, but from what I'm reading it sounds like maybe Uru comes chronologically before End of Ages?

Yup. You could go Uru, 4, 5, or you could go 4, Uru, 5. They're pretty standalone so it doesn't matter too much, but you'd want to save 5 for last because it will fairly casually spoil elements of the previous games.