Having played and ran many games with multiple different versions, it mainly depends on the game itself.
Shadowrun:for the most part every new edition has been a great improvement on the last (though I have 4th and have thumbed through it,never played it or read it cover to cover yet so no judgment on that from me). Still I've never had a complaint about their new editions.
Star Wars (d20):there was much moaning and gnashing of teeth when RCR came out, but ultimately most people around here finally adopted it because it was a better product. Of course the reason for that was because the original SWd20 was so bad.
(While I have Saga, I haven't read through it yet,much less actually played with it)
Traveller: An example of what not to do. Between rules & settings the fan base is completely fractured,if someone invites you into a Traveller game with no further info, you can't even plan out a character, much less create one, till you get there. A lot of it is due to the timing of everything, GDW dissolution while still trying to grow T:NE, followed by handing out the IP to pretty much anybody who asked. While it did breath life into a dead game, were the results truly the best that could have come about? Hopefully MGPs "One true book to rule them" will consolidate things back together, though only time can tell.
Traveller is also worth mentioning because it serves as a warning of what one of the worst case scenarios could be: IF D&D 4e flops and the fan base splinters, some playing 4e some playing 3.5, some playing True20, and so on, it will look a lot like the current Traveller community (heck just replace era with setting and you could even have the same arguments).
OWoD (Mage in specific for me): The 1st eds were buggy, the 2nds cleaned many things up such as foci, sphere power levels,though also made some subtle and unwelcome changes to the rules (I LIKED the magic hip flask and the third rail as examples of how coincidental magic worked and disliked seeing that style go). Now Revised was an entirely different beast altogether. A few good rules changes (half of which were common house rules already or even optional rules in the Player guides), coupled with several unnecessary bad rules added (like the new resonance mechanics-useless and actually unrelated to how or what resonance was in previous editions). But what people really disliked were the sweeping background and setting changes.
NWoD vs OWoD: Keep in mind that these are entirely new (and basically unrelated) games. White Wolf wiped the slate clean and started over. And to some extent this is what WOTC is doing. Just keep in mind the difference that most people play Vampire: the Masquerade or
Were Wolf the Forsaken NOT WoD as a game, while on the Wizards side most people play D&D, perhaps set in Eberron or the Realms or Living Greyhawk or even some home brew, but the still view themselves as playing D&D.
Then on the opposite end of things there's Palladium. While they have done 2nd editions of most of their games, the improvements have been minuscule, and in the case of Rifts most of it was left alone specifically to keep most of 40 odd source books valid,leaving the power creeps and troubles intact. And if any game needs a Monte Cook/3.0 style ground up examination and wiping of the board, they surely do. Of course since at heart its nothing more than someones OD&D home rules, would a modern rewrite get you something different than d20?
While I could go on and on about many other systems I've played through multiple editions of (Deadlands, MEGS, Exalted,L5R) these help illustrate the real question: How will the fan base react to 4e? I can't speak for the fan base, but I'll judge it on its own merits. If its truly a better game or the game I want to play (not necessarily the same thing), I'll go ahead and pick it up. Otherwise, as you might have guessed from the preceding paragraphs, I have plenty of other games to choose from.