I'm a recent convert, but had a lot of trouble to begin with. Having said that...I'm glad I stuck with it....it's such a good car once you know how to tweak it.
First rule: Keep the slipper really tight....tighten it fully and back off 1.5 turns at the most. This will be fine for most tracks. If you let the slipper slip it will just heat up the plastics and stuff will melt. The drive train runs so fast inside the buggy (really really low internal ratio = fast!) that it generates heat quick. Keep the slipper tight and let the belts skip and you'll be rocking.
Second rule: Reinforce your shock towers with CF from penguin r/c or dc-racing. I broke loads and loads of shocks since upgrading to the X-5. Theoretically this should have been no different to the XX4, but I think that because of the extra grunt from the low internal ratio I was unable to handle the power and it resulted in some unusual collisions and big air!
Third rule: Fit the belt tension mod from 4wdrc here:
http://www.4wdrc.com/tech/BeltAdjuster.htm
This makes getting the right belt tension really easy.
Fourth rule: Get the upgraded machined pulley gears from Lesro. I think these now come as standard in the new kits though, whereas they didn't when I got mine.
That's all I can think of tight now.
I'm sure one of the X-5 crusaders like Chris Long will reply to either add to this or put me right
. He's been at this a lot longer than I have!
Once you get it right though...this car is very rewarding to drive.