So had a chance to play several games over the Holiday. Voidwalker novabomb spam aside, I think the issues with supers are still somewhat present from D1.

Hunters:

Ark Strider still suffers like blade dancer from the need to be close, slow speed, and minimal armor. Without any maneuvering bonuses and with less then stellar grenades, its very hard to get reliable kills. Players can still jump up in the air and avoid or confuse the ark strider targeting, and in mayhem the other supers can easily be deployed to shut down the ark strider super with minimal risk (with the exception of golden gun, which lacks any armor and has a long boot up).

Getting kills with ark strider was the most difficult by far of the three hunter classes.

Gunslingers:

Golden Gun is excellent, long ranged, and with the fix to the 6 shot variant capable of cleaning up a group. The downside is still the lack of armor, so it's useless close in.
Scatter mines seem to be the grenade of choice. Trip mines no longer killing are far less useful, and incendiary grenades are hard to hit with.

Nightstalker:

The king mayhem class, with constant invisibility and a long range super. Smoke bombs and void grenades can kill a weakened roaming super class, and the tether is an excellent shutdown super. This is the new anti-fun police build of D2.

Titans:

Strikers:

The best mayhem super. I spent a whole evening with Lion Rampant just boosting towards the other team and using fist of havok, and managed around 18-22 kills a match. It's brain dead easy. Extra armor on cast, near instant aoe damage, and hit hits people jumping above you. Lightning grenades and pulse grenades are both good options that I could leave lying around for people to walk into.

Sunbreakers:

Do you envy hunters, with their long range GG shots? This is a close second. Again pair with Lion Rampant and live out your AC 130 fantasies by hovering high above and raining explosive mortar fire below. Your grenades may not be very good, but you have high armor in super, enough to even trade with strikers on occasion, and the extra explosions are great. enemies will have to jump aloft to avoid you, and this is the only major drawback to the class.

Sentinels:

On paper, I had assumed this class would suffer like the Ark Strider, but the difference is that it has the ability to toss out the shields for a ranged attack, and shield themselves on the way in. The suppression grenade is hard to hit with but can be good when it does connect. On the whole it's the hardest of the three Titan classes to use in Mayhem, but is significantly easier then the Ark Strider to use.

Warlocks:

Dawnblade:
This one is fair to middling. The single target slow firing and the need to hover make it very vulnerable. You can out run a dawnblade, or counter with your own super most of the time. Likewise the grenades are not too great.

Stormcaller:

You need to use Ionic blink. Otherwise your asking to be shut down. Going landfall can get you the D1 striker feel, but that's about it. On the plus side you don't really need to aim nor get too close to the other team.

Voidwalker:

Blink + Slovabombs are probably the 2nd best mayhem combo. You have to think more about the positioning and setup to get multikills, but the secondary void bombs spawned plus the large blast radius are excellent. You have lots of exotics that pair with the class for added melee range, damage, or ability recharge, not that you need the latter in mayhem.
Guided novabombs seeking flying Dawnblade or Sunbreakers is classic fun. It also clears those teams that tend to camp in back rooms by a heavy spawn nicely.