My feelings are kinda mixed. The Advent have the best sounding voices of the lot, though they're not really my playstyle, they're the least offensive one aurally.
The TEC, on the other hand, have the 'worst' voice acting, in that it sounds a little inconsistent. Most egarious of which is probably the Cobalt frigates excessive... eheh, enthusiasm, followed very closely by the Marza - it's just too loud and brash compared to the others. The scout frigate also annoys me a little, but for entirely different reasons - you're a minion sent to scurry around, not some hotshot special ops dude! (There again, it could be kinda cool if they were... I wonder how you'd rig that? The explosive charges are in the right direction; pity they're useless in a game with no form of stealth
)
Ironically enough, the Vasari voice acting comes somewhere between the Advent and TEC in terms of consistent quality - and yet, the style they chose for them means I play them sparingly, when I can stand it, because they get on my nerves too fast. >.<
I think a more whispery sound set could have worked a little more smoothly than the chainsmoker aesthetic, but that's just my personal opinion. 