I can run the game with generally highest settings if I turn shaders to low. I don't know why but as soon as I pump shaders up my FPS takes a gigantic nosedive more than any other setting. Also I'm hearing that it's best to disable anti-aliasing for now as there's an uncommon memory bug with it that can degrade performance after a few hours. If your game starts to shudder after a while, exit and restart it and that should clear it up. YMMV.
I'm a 23 Marskman and loving it. Definitely get a multi-moded Zeus or assault rifle and stick to upgrading that to make things very easy mode. Sniper skill is also very great as the assault/zeus type rifles are compatible with it, doubling damage and greatly increasing their ranges.
Here's a short video I made for a friend to show some of the weapon types marksmen have. It has no production values and I blindly picked a song. It should give those on the sideline some idea on how the ranged combat works (which feels better than Tabula Rasa as it's very much like playing a FPS)
hl_0001.wmv - FileFront.com
Marskman or summoners with elemental builds should clear the normal game without too much trouble, clear it on multiplayer normal and start an elite/nightmare mode with a more challenging class if you feel the game is too easy right now.
The game does have its flaws, but mostly engine bugs, and not crippling performance issues or fundamentally flawed game design. It has good potential if they can get out of this initial launch hump.