I have been working my way through all of the /time masterminds, but I haven't gotten one to 50+, and have not gotten to Mercs.
Ninja/time felt the best out of all of them. Most postings about ninja are from before the patch, so all you get is repeated tissue paper comments. After the patch, they have 13-18 defense, granted by the first training power.
Thugs/time was good, but didn't feel right. To me, /time needs a melee primary. That said, it will probably be the first one I take all the way to 50.
Beast/time was interesting, but the attacks were slow.
Necro/time was pretty damn amazing. The addition of the ghosts was huge. They didn't do a lot of damage, but could tank, so no individual zombie got dog piled.
Demon/time. Again, it felt like it needed melee to keep everything in Time's Juncture.
Bots/time was ok. Nothing special, nothing terrible, compared to the rest.