I've had both Losi RTR's (8B and 8T 2.0's) and they were awesome. Only REQUIRED upgrade, in my case, was the steering servos.
I slowly upgraded parts until they were essentially rollers - supporting the argument that over time you end up spending a boat load more money than if you originally bought the roller.
However, you can learn about rebuilding shocks and diffs just as good w/ a RTR as you can a kit. You end up breaking the car down (or I argue that you should) when cleaning, doing maintenance, or adjusting setups. Therefore, learning how the pieces work and go together just as if you built the car from scratch.
Losi RTR's are of great quality and will not short change you of the joys and experience of nitro racing.