I recently fought in a team deathmatch missiles game against two other raiders on the other team, both of them performed fairly well. They certainly outperformed me.
I think raider is viable, it's just really dependent on aim and landing your corsair shots. The secondary is really essential to fights and missing shots with such a slow fire rate can make or break you against a mech with tow launchers or machine guns. Usually if I missed a corsair shot or two I lost the fight, if I landed the shots I won because corsair can be pretty powerful. Also closing the distance and being good at close combat-which I'm not-- is required, because raider pretty much only has close combat weapons. The reflak's ricochet is useful, missed shots can ricochet back and make up for the miss. It's also surprisingly effective against heavies in fortified mode; shoot the wall they have their back against and pelt their weak spot. Team composition and objectives also effected my performance, if the enemy team kept together and focused on killing rather than an objective it was harder for me to flank enemies and usually I'd just stick with my own team and attack head on.
I'm gonna try to upgrade my graphics card at some point, I'd imagine the better FPS will improve my aim and movement. The game still runs waaay better than beta, don't know how I could play during beta testing. The game could run even better though, and I'd love to see nicer graphics than "low" setting.
Anyway, probably will try a grenadier or scout, maybe predator. Grenadier because I love projectiles, and scout because I love being fast and the raider's flak shotties grew on me. Predator seems interesting because of the cloak ability, and the whole "mine trap" utility sounds like fun.
Now I just have to save up the hawken points.