Eh, It's a very mixed bag. I play (and run) Living Greyhawk games on occasion, and I've been in RPGA games that were great, and RPGA games that were terrible.
The people that play in the adventures are often nice people who are fun to play with, but there are also a lot of over-the-top min/maxers and downright cheaters--Because the individual DM running your game probably isn't that familiar with your character, you have a lot of opportunity to bend and break the rules when a DM's knowledge of some specific tactic is fuzzy.
Plus, whenever you run into genuine optimizers, you also run into people who think they've discovered ways to cheat the system, but are really just wasting everyone's time with tactics that look impressive but are no more powerful than a standard attack.
The adventures themselves are often a mixed bag as well. Firstly, they're often uninspired, railroady affairs--One adventure that was specifically described as "non-linear" merely presented the same four fights in a different order based on one choice that the group makes at the beginning of the adventure. Once the adventure actually started, they could make no decisions about what to do or what pace to do it in.
Secondly, they're often set up with the worst players in mind. That means A) The roleplaying is optional at best and nonexistent at worst, so that the folks who can't roleplay themselves out of a wet paper bag can still handle the adventure. and B) The encounters are usually set up to challenge the Min/Maxers, so if your characters aren't optimized, your in for a tough (and usually unsatifying) slog.
Thirdly, Adventures are usually intended for a broad range of character levels, but the authors don't really take into account what variously-levelled characters are capable of. In some cases, characters can break the adventure simply by using the high-level spells they've earned access too. In other cases, characters are put up against foes they simply lack the resources to defeat (DR + fast healing+flying vs. a first-level party, permanent status ailments before the characters have access to curative magics that can remove them, etc.) And in some cases, the combats are just plane ridiculous because the high-leveled characters are reverse-engineered (One adventure had characters going up against a mage in full plate--At the higher APLs, he had some ability that let him cast spells while armored, but at that level, his spells just failed half the time.)
Really, There are only 3 reasons to Play Living Greyhawk:
1) Not enough players or not enough time to have a regular game. RPGA's biggest strength is its ability to bring random strangers together for a game. And even if you've got a group, RPGA can help your GM cut down on the prep time. RPGA is a vast source of free premade adventures
2) GM sucks (for whatever reason). The highly-structured adventure format that leaves little wiggle room can be really helpful for the novice (or merely crappy) GMs. In fact, running RPGA adventures is a good way for a novice GM to beef up his rules mastery and table management skills.
3) They can be fun. More that half the time, RPGA adventures I run or play in end up being mindless combats. As someone who doesn't even like mindless combats, I end up walking away from the table with a smile on my face 70% of the time, simply because I'm playing D&D with a good group of people. If you do enjoy mindless combats, then you'll probably have even more fun than I do.
(Note, I only ever play in Living Greyhawk Adventures (most of which are Nyrond Regional or MetaRegional adventures). I don't know how similar the adventures I play are to adventures from different RPGA campaigns (or even just differend LG regions).
