I own a lot of the games on here already in original disc format and probably could get them to work ( via dosbox, scummvm, etc) but are they optimized in some other way to justify a re-purchased? For example I have all the Tex murphy games on sale this weekend and can usually get them do work when I want to play them, but need to swap discs every few minutes. is that sort of thing now gone, are the crashes less frequent? any info on any of this would be much appreciated. Thanks

Yes, that is one of the major benefits of buying games from gog. The work is done for you to make the old games work well on a modern machine. I haven't had to fiddle with any game I have purchased here, except ProPinball: Timeshock, which is still stuttery. Still that's one out of 50 something games.
Also, you will find that your games often come with lots of extras, like downloadable soundtracks, wallpapers, artwork, etc.
Plus, since it's DRM free, it's easy to copy and backup your games with all the DOSbox / SCUMMvm work already done for you.
If you need a little proof, there are several free downloads on the site: Beneath a Steel Sky, Teen Agent, and Lure of the Temptress are all free.
Welcome to GOG! :)

You won't need to swap discs for a start as all the disc images are premounted into the dosbox config. That said some of the games do have problems, not everyone is going to experiance it but some do, so I would advise checking any individual game forum before purchase to check the current situation.
I think all the dos games run in dosbox so for those its just a preconfigured setup, game patched to the latest version that comes in a nice easy installer.
For windows games some of them have had alot of optimisation just to get them to run in xp or vista at all.
Alot of the games come with extras as well if thats your sort of thing, what extras can be found on each of the game pages.

For one, DOSBox or ScummVM (whichever's needed to play) is already included and set up with the installer, so there's no need to mess around with the config files. As for disc swapping... well, you don't get any discs, they all run directly off the hard drive.
Everything should run from the get-go on Windows XP and Vista, also 7 even if it isn't completely tested and officially announced yet.
Frequency of crashes varies between games, but should be minimal.

I can't speak for the Tex Murphy games, but for the games I have bought through GOG that use DOSBox or ScummVM they work quite well and with pretty much no hassle. Install the game, run it from the provided short-cut, and that's that. The game runs, usually without any crashes or glitches, and without requiring tweaking any game settings or getting under the hood to tweak DOSBox settings or the like.

Thanks for all the, answers, but I think you should know I have already gotten games off of this site before (games that I did not already own but had always wanted to try) What I am asking is it really worth re-purchasing games you already own (in their original format when they originally came out years ago) just to save some hassle?

I think that's a question only you can answer, as it really depends on how much you value your time and how big of a hassle it is for you to learn to use dosbox and scummvm.
I will say that one benefit is that since it is all packed into the installer, it's nice to have the gog version because if you ever have to reinstall from your backups, you won't have to go through the process of making them compatible a 2nd time.

There's no definite answer to your question, some will say it's worth it others will say it isn't depending on a few variables, like how much do you like the game in question, what extras come with gog's version, how much of a hassle it really is to install/run the retail version, how much do you want to support gog, etc.
For me, it's definitely worth it. The 3 last games i got from gog are games i already own in CD/DVD version (Myst ME, RealMyst, Riven )

I already know how to use both of them (I use them both frequently, just some setups don't go as smoothly as others), I said so in my original post, what I want to know is whether or not people who have already re-purchased games they already own on GOG have noticed a huge improvement in their quality of play over what they themselves could come up with when try to set it up by hand

I'm probably the wrong guy to ask mate, as i'm a bit obssesed with the series, and i consider Riven to be the best of the lot, so my opinion might be a bit biased ;)
But i think most will agree it's a very good game by any standarts.
Personnaly, i love it.
Edit: Oops, didn't catch your edit before posting. Any way, there it is.

If I can load and run a game in XP or Vista its fine for me no matter how old it is. I have no problems with the Baldur's Gate Series, Planescape Torment, Icewind Dale, MM 6-9 and many others. Which is why I don't buy games from GOG like Divine Divinity or the Gothic games - these all load and run fine for me. If a game has issues I don't want the hassle of messing with getting something to run otherwise. For the 5 - 10 dollars it is worth it to me to buy it from GOG and know it will work without a problem. I don't think I have purchased anything from GOG I don't already have a full boxed version of except the Ishar Series.