He sees a decent amount of use for me, ensconsed in 4*-land, but with a good number of champed 4*s now. He can be handy against the extremely common Grockdusa or Gritty teams in particular. One thing I've done against Grocket + Bishop + Peggy in Simulator is Grocket/Dazzler/DP -- Dazzler is the important bit for the first turn (make your first match with Dazzler, then Bishop takes the hit, hits Dazzler back, DP gets a retaliation, and Dazzler wipes out half the opponent strikes and stuns Bishop for 4 or 5 turns, enough to kill him with the strikes of your own) but DP is really good to help with the later turns -- his countdowns count for Dazzler's green, and his red is a big finisher. Not great on defense of course because the opponent doesn't have to hit Dazzler.4* Deadpool is solid, and he occasionally shows up even in 5* teams. His red does good damage and heals him, his purple does decent damage and denies the opposing team their strongest color, and his passive means the enemy has to attack him or take a lot of damage.
I'm not sure how strong he is on an all-4* team (though I remember him as being quite good), but he keeps his value much longer than most 4* when you're into the 5* transition.