There are usually considered to be OK, but nothing fantastic. Offhand, I don't think they use an individual reflector for each bulb. This does cut down on the amount of light you get out of the fixture.
Your obviously going to get cheep bulbs. Compare the bulb replacement costs to the price of the fixture. Corners had to be cut someplace to get to that price.
That being said, the price is low, so you don't have much at risk if it doesn't work out. Also, if there is a problem, you can usually repair a T5 fixture with off the shelf parts.
Personally, I'm not a fan of Ebay, and never use it, but others feel differently.