Its pretty sweet once you get the energy management down. A nice approach and even an classic lerk makes mincemeat out of 1-2 marines before they get all that many shots off - good for early game containment. Especially since the tons of medpacks thier comm would have to drop vs spores will hurt them at that time.
Of course, for a 30 res investment he's fairly frail too but not too bad to not be able to survive. Towards the mid-game he becomes superb with Umbra/Adren or Celerity. The approaching is not that big a problem - most people will just fly too low, or frontally - obviously, you want to either have a reasonably high ceiling to drop down from or approach from the back if you're going in for a bite on multiple 'rines. If you absolutely have to approach from the front, pull up towards the ceiling as the Marine gets his aim on and flap 1-2 times to keep that speed up - vtracking as we well know tends to be everyones weakness, and its easier to get away from a fight if you don't bump into the guys on the ground and loose your speed.
Thats really how 90% of my lerkdeaths happen. Run into a guy, loose all speed - and you're already low on adren. Erk. The other 10% are 'nades :p
Anyways, my favorite alien in all honesty. And in co_, its just plain insanely good (Cel/Adren/Regen/Feces/Carapace/Umbra = win). But Celerity alone is enough if all you want is some fun flying.