I've never played one, but I've wanted to, and therefore looked carefully at the class.
Why did I want to play it? Our group doesn't always have people show up to all sessions, so I felt like I could cover a variety of "positions" on a day-to-day basis.
The major "abuse" that you can accomplish with a Chameleon involves a lot of down time -- basically, you use your floating feat to take item-creation feats, and craft some good stuff. You need a previous spellcasting class to get access to item-creation feats though.
My most abusive qualifying pre-Chameleon build would have been Rogue 2 / Ranger 2 / Cloistered Cleric 1 (with Practiced Spellcaster: Cloistered Cleric), and I'd play a back-up Archer & Healer until I got access to Divine Power. The module was low-wealth but high-death, so I thought the abusive item crafting trick would have been appropriate, if we got the downtime.
(Ooo, another fun trick is to have feats that boost a specific class feature like Favored Enemy, and use your floating feat to pick Extra Favored Enemy against whatever foe you expect to see a lot of today.)
IMHO, it's a fun & interesting class that allows a group to compensate for missing members more easily.
Cheers, -- N