I'd third the options listed above, it adds some reasonable backstory to the game. It also adds some good weapon mods useable again on NG+, if you're going back for a second go-round.

I'd suspect it'll go on-sale when the Omega DLC comes out, though if you're not hardcore ME fans, wait for the sale.

I enjoyed it and though it was second to LoTSB in quality and enjoyment value.

My DLC rankings across ME1, ME2 and ME3, from best to worst:
LoTSB
Leviathan
Bring Down The Sky
Stolen Memory
From Ashes
Overlord
Arrival
Price Of Revenge
Pinnacle Station
Firewalker

I think that covers all of them.