My experience is limited so far. I've only been playing since Friday, played around with a few of the classes and got some of them to level 2, part of the way to level 3. I've also played mainly on normal difficulty because most of the time I'm just pubbing but I've had some good games on hard with friends.
My personal experience is that after the first 4-5 waves, the berserker completely runs out of steam whereas other classes move from strength to strength. Once everyone has a decent weapon, most specimens are getting dropped before I can get near them. On wave 1-3 I can wade into crowds of clots with my trusty knife and decapitate them all in seconds but as soon as crawlers start showing up in large numbers, I probably spend more time avoiding them than killing other specimens. Sure, I can tank a skrake a bit better than the other classes but why even bother when sharpshooters and demomen blast them into small pieces?
Lets talk about the weapons. As a level 2 berserker, I feel like I peak after I buy or find a fire axe. The Katana is better on paper but I'll be damned if I have to click every single time I want to swing the bloody thing. The Chainsaw seems like a joke and a liability when faced with groups of enemies. I decapitate a bunch of enemies and... oh wait they're still attacking me. Brilliant. As my team mates buy bigger and better guns, their killing power increases exponentially and mine decreases. Once we get to wave 5-6+ I find myself buying something likea bullpup and simplying using the zerker perk for the clot immunity, and melee weapon speed to get around the level quicker. If I try to go primarily melee, I don't feel like much of a credit to the team.
TL;DR from the perspective of a new player, the berserker seems lacking.