As an almost complete newbie, I thought I'd give my .02.
I picked up a brushless B4.1 RTR a couple of weeks ago. This is the first R/C car I've owned (outside of toys as a kid). There are definitely pros and cons to the RTR vs the FT Kit
I didn't build it, so out of the box I didn't know exactly how it all went together. However, I've had to fix a couple things since, and I spent a ton of time looking over the diagrams to figure out how it all works. So I don't really feel like I lost out on the building part that much. Being pretty mechanically inclined, the vehicle itself wasn't much of a mystery,
I also tend to read a ton about any new thing I try, so I've spent hours online reading about assembly, tips and tricks, setups, etc. I may not have done that ahead of having the vehicle in hand, but it's hard to say. I did enough reading to know which RTR I was going to get.
Looking back at some of the upgrades I've made already, I probably would have gone with the Factory kit and built it up. But I really did like the fact that I left the LHS with the car, charger, and battery and I was running it around in no time.
The biggest drawback for me personally is that I've already upgraded, or ordered upgrades, to the big ticket items - ESC/Motor, Radio, etc. SO ultimately it would have been a little cheaper to have purchased the FT kit outright and bought all the things to go with it. However, having the RTR in hand, having fixed some things, and sorted out what I wanted to upgrade was all easy since I didn't know anyone with some cars I could try out. For the average person I'd see the RTR as a great way to see if they were going to stick with the hobby...
Mike