I think your reception of 4e will largely depend on how you played 3e. For me and my group, 4e felt like a natural extension of and evolution from the 3.5 ruleset, with supplements. For others, it's a massive departure and doesn't resemble any D&D they've ever played.
If you tend to run a fairly encounter-heavy 3.5 game, and if you use a lot of the later supplements like Bo9S, Complete Mage, Complete Champion, and so on; and if you usually run encounters on a grid ... 4e will feel a lot like a natural evolution of the D&D you're used to.
If you tend to run a very low-combat 3.5 game; or else stick to the core 3 books, and don't care for Bo9S-style classes, reserve feats, etc.... You're more likely to think 4e is an enormous departure. (You still might think it's a fun game in and of itself, but you probably won't be able to shake the feeling that it's not quite D&D.)
These are gross generalizations, mind you. There are always exceptions.
For me and my group, 3.5 was feeling a bit stale. As a DM, I was disenchanted with the prep-work. I was also getting progressively less happy as combats took longer and longer for the same amount of things happening. 4e worked great for my group.
I don't think 4e is a good choice for lots of other groups, though. As an example, another game I'm running is Call of Cthulhu d20. Even though it's still class-level, skills are really the focus of the character system. Simply put, I don't think the 4e system of Trained/Untrained, slowly-increasing skills would be a good fit. So, by extension, if I was running 3.5 the way I run CoC d20, I would not like 4e. On the other hand, I love skill challenges, so I imported those from 4e into CoCd20. (FWIW, I think you
can run skill-heavy games in 4e, but I don't think it's a perfect fit.)
Basically, I'll echo what everyone else has said - give it a try with the free KotS download. I'd advise using the free Character Builder rather than the pre-gens, but it will work either way! My only other advice is to try and separate the system from the module, if that makes sense. It's not the greatest adventure ever written, IMO.
EDIT: I should also note that I'm not a guy who finds one good game and sticks to it for everything. I look at RPGs as just a bit more of a commitment than a board game... I don't expect every RPG to provide every kind of experience, or to be the best at everything. I have found 4e works great for the kind of
D&D I like running, but then again, it's not the only game I play. As I mentioned, I also run CoC d20, SWSE, WFRP2e, Paranoia, and other games as the mood strikes. 4e is, as of right now, a lot less of a world-building toolkit than 3e is. This isn't to say it's inflexible, or that world-building is impossible (far from it!) but it is focused towards doing a
few things
very well rather than a lot of things simply well. Again, this fits into what I want from an RPG nowadays. However, if your group is a single-game group, you might find 4e lacking in ways that are important to you.
-O