Both advanced classes of the bounty hunter are viable and competitive. Powertech gives you the option of solid dps or tanking, and mercenary gives you solid dps again, and healing.
End-game options for solo pve are quite limited mind you, you're going to be mostly restricted to dailies. Group pve is where its at, and is the most fun in my opinion.
I found solo-pve with a tank spec a little difficult, as a healing companion has difficulty keeping up. But of course, you have equal dps specs on both sides, so it really doesn't matter.