I have always been annoyed by him and rarely use him. I even avoid him in Arena. He is certainly OP at least until the update with most of the top 20 running the old guy.
Well two days ago I swapped in my 5* Dooku as lead on my Arena team. I did it on my first battle since I thought he would be just "OK" as 5* gear 8. I fully expected to lose and I could take him back out for my next 4 battles. Since doing the swap Arena has gotten 10x easier! Even at 5* he is workable with gear 8 and all abilities maxed. Most battles I am winning with 4 toons at max health. I also found my rank seems a lot more stable.
Granted I am just ranked around 300-ish but my team is also pretty weak.
All level 70 - Dooku 5* (L), GS 6*, RP 6*, Sid 6*, Lumi 6* - all max geared except GS, he is gear 8.
The Offense Up on evade works wonders with RP and GS and Dooku can stun QGJ or Lumi out of the gate so I can take down Sid or a cannon without worrying too much.
I still dont like him and I am mad at myself that I havent used him earlier but I admit to being impressed.