Just got Havok, and since I am also levelling Psylocke and other heroes at the moment, levelling Havok has been slow.
Have all heroes other than MB, with Daredevil being the only one at level 10 (Psylocke is at 6, training to become 7) and a slew of others at 11/12. So unless I get Magneto, I'm looking at 24~36 hour training times, meaning Havok will probably gain a level only once every day or two.
So far, Havok has been quite underwhelming (lvl 2). His base accuracy, defense, evasion are all unfathomably low, and since my agent is at level 240, he misses. A lot. Being a blaster, he's also fodder at 10.6, making him quite painful to level. I may train Daredevil the next time and start farming 9.6 instead since it's a bit more blaster-friendly.
He will probably get better at levels 3~5 as I can equip him with some +accuracy crystals and once I start using him in missions with energy attacking opponents. Paying attention to stamina to take advantage of scaled damage for his lvl 2 attack (later lvl 9 as well) will be something I'll need to get used to.
Of the slew of new heroes I've recruited, namely Beast, Valkyrie, Tigra, Cable, Hercules, Quicksilver, Psylocke and Deadpool, he is currently dead last in terms of satisfaction. Of course this will change once I get him to level 9 and get a feel for his full arsenal, but I really didn't like him as a team-up either, so we'll see. Then again, I absolutely hated Valkyrie when using her for team-up but actually like her a lot now.
Ultimately it will depend on how he gets slotted and how his final stats end up. But as of now, I'm not impressed and he's just causing too much trouble ;)