You will probably find the set bonus of the Temporal Warfare set much more interesting. It's generally agreed that the Jem set upgrade isn't really worth it.
the last couple content patches of the anniversary event starting seems to have buffed it a bit. i think its worth it. I got it on my Dread carrier running phased polaron and dominion polaron wepaonry. i plan on saving up for Elite Fleet polarons and equipping those. in full attack power my dual phased polaron bank does a descent amount of damage while running Attack Pattern Omega on my eng.
while it's not an 'end all' set, when on dominion ships i think its pretty good. the polaron power bonus for mk XII was boosted to 14.1% increase, and the weapon power bonus from the engines was boosted to +10, which means when put on a jem bug you have more power to allocate to your other systems.
Joined Sept 09.
Curse you Archived Post.... you stole all my Ideas.
when i got my fleet saber, I slapped the mk11 jem space set and started playing with various polaron builds. I did upgrade it to the mk12 set recently and I have to say that my survivability has increased and I play mainly PVE content, heavy on the STF's. I also use three of the dominion polaron cannons (blue mk11's obviously) upfront and they are more effective (best word i can use) than my mk12 purple Borg Polaron DHC's.
I had 300+ I wasn't going to spend on anything else, so upgrading the sets made sense. The JH space is nothing that impressive, but if you're just getting to 50 and you don't *have* any of the STF / Reman stuff and can't afford or craft Aegis, it's not bad. It works well with the polaron weapons, it's a good damage boost.
The ground set does have unique armor look which some people might like. The health boost and occasional stealthing can come in handy.
Long story short: If you have a full MACO/Omega/HG set on ground, the ground really offers nothing. If you have a complete MACO/Omega/HG set on space, it'd debatable, it's not really good for Borg but works well on fleet dailies and other stuff.
If you have no end-game sets, it's well worth 300 lobi.