I CK Castles and Crusades. I started gaming in 1982 and quit in 1993. I sold all my stuff in 2000. In 2003 I decided I needed a hobby, so I got back into gaming. Bought the 3.5E Players Handbook, Monster Manual, and DM's guide. Got a group together from the local college, and started DM'ing again. We play every Friday. When I first encountered C&C I thought why? But bought it anyways. In the meantime, I was getting burnt out on running 3.5E. Man! Prep time is a bitch, levels come fast, and the plethora of special abilities, feats, etc make it hard to maintain a challenge game over a long period of time without MASSIVE prep work. With all the options, IMO, it limited the players thinking. Then the Monsters and Treasures came out, and I got it. I started 'hanging' out at the C&C boards. Great community! So one day I had enough of DM'ing.
I was not liking my hobby anymore. Either the hobby had to go or something had to change. All of my players were die hard 3.5E'ers. But I sat them down and said I need a change, this is what I want to do and why. They agreed to try it. We converted the characters over and started using C&C. The players have eventually learned to really like the seige engine, the different way spells work, and the difficulty of gaining levels (They are more of a treasure now). The best thing they liked and I liked was how much we got done in sessions. I got a new player who has DM'd 3.5E for a while. After her first gaming session with us she said 'Wow! We got more done in this time than we ever did with 3.5E! No miniatures, no rules arguments, just gaming!"
The characters are as 'different' as they where in 3.5E, they just don't have 50 rules defining how they are different. I have one player who has a Half-Elf who is a Fighter/Rogue. He said hey I really want the ability to many shot. Fine, we worked it out. The wizard in our low level campaign is going to find an ancient text describing how to 'empower' his spells. The game is really only limited by your imagination.
Before trying C&C, our group gave several sessions to True20, but in the end we scrapped it because of the wounds system. Something about rolling for damage vs. making a toughness save. We all felt it was 'wonky'. The character creation system, however, is probably the best I have seen. We thought about adapting it to C&C; we still may in the future.