The OP said he already has the Uncharted series, and I heard mixed reviews of GT5 (never played it myself).
If your interested, go read Popular Mechanics this month, there is a list of championship drivers who "started" on GT series and all claim it made them better, and more info on how several big name race shops are using it for training successful drivers. If you want to dig deeper there are several stories (mostly in European racing mags) on how its the most realistic driver.
GT is one of the BEST racing simulators period, bugs or no. Is it the most fun? depends on what you like, pure simulation or semi arcade racer.
personally love the GT series but Motorstorm (ps3 only) is the one i like for just fun.