Yep, very useful, which is why he's on so many teams. Not sure who are running, but I usually focus him down first in GW, because I aim to take the same 5 guys through GW and having a heal block on them is deadly.
Having fast characters with extra attacks and status effects helps a lot. My GW team now includes Dooku, Daka and GS, usually. Between them, Sid gets one attack on me and he is toast (unless there's someone much more dangerous. If I can't kill him fast, I stun him. If I can't do either I swap out GS for my Sid and try to beat him at his own game.
A counter-attacker like Dooku can help a lot because he likes to AoE early and the counters can knock him down far enough that a striker can take him out.
