Single player is kind of meh in general. I found it to be little more than a shooting gallery, complete with endless waves of brain-dead enemies that seem to pour out of the cracks in the floor. One level features a construction zone with a cement tube suspended way up in the air by a crane. When I saw enemies perched up there I just started laughing.
The regenerating health bar just doesn't work for me in single player mode either, I find combat has no tension when I know that I can just hide for a few seconds to heal. There is a mode with an actual health bar, Classic mode or something, and that can be a bit of fun (I played one level with it). But I think you have to beat an easier difficulty to unlock it.
More on topic though, I hate the grenade launcher. I don't know why, but tubers tick me off much worse than Masties or Proxies. Instant death and they only need to hit near you.