I've ranted on about flash gitz in other places, but all in all, I think they're an interesting and frequently useful unit. They're just not practical.
What makes them frequently useful is Kaptin Badrukk. He has the most reliable AP 2 shooting in the entire Ork army, especially considering the fact that he comes with free ammo runts, in addition to the ammo runts the Flash Gitz can take with the unit. That's a lot of firepower. Yes, they're not cheap, but they're an interesting alternative to nobz.
That said, they're not practical for three important reasons:
1. They just don't have the aim or number of shots to outperform regular shoota boyz, point for point.
2. They don't have many of the normal nob options, including powerklaws, waaaugh banners, bosspoles and such.
3. They can't take a dedicated transport.
The last reason is the big one. They don't really work on foot, so you need to give up two heavy slots in order to use them.
However, I feel they can still be useful and fluffy, especially in apocalypse games, where there's no force org chart, and you have a lot of points to spare.