Flash Gitz aren't bad, properly supped up, they are very resilient as far as orks go, and only moderately shooty, and barely average in assault (as far as orks go anyways).
But they don't really excel in anything save resilience, and they aren't cheap enough for purely that, nor is pure resilience an overly useful trait in 40k. Especially with the number of point weaknesses the Flash Gitz resilience has.
In a raw breakdown, out of 5 stars
Assault : **(***)
- 2 in relation to the rest of the army as a whole, 3 in relation to a raw empirical base
Shooty : ***
- 3 Decent range, decent profile, and assault qualities would make them 4 or 5 star, but being Orks, their BS is horrible and that brings them down. The random nature isn't bad, but it doesn't lend too much to the unit as it's by no means reliable to any standard.
Resilience : ***
- Only a 3 actually, against small arms fire, they are naturally moderately resilient. Against heavy weapons fire, they drop just as quickly. When fully upgraded, they become effectively a 4 star against small-medium weaponry, but still die just as quickly to heavier guns. In all cases, their poor leadership qualities subtract from their resilience, and the proclivity of cover boosts cheaper units
Cost : **
- For what you get, it isn't bad, but to get it all, it becomes quite expensive and still doesn't provide that much. They really aren't anywhere near as cost efficient as Boyz, even with minimal if any upgrades, the synergistic effect of Boyz outweigh the unique abilities provided by Gitz.