Felguard is excellent for solo, but not until around 60 do you get enough talent points to really use him to full advantage. I leveled my warlock as Affliction until 60 (which was right after BC arrived), then respeced for Felguard. It certainly made solo leveling easier in Outlands, but I ended up re-re-specing back to Affliction for end-game. Trying to micro-manage the Felguard during boss fights just wasn't any fun, and my personal DPS was so bad my first Shadow Labs run couldn't get past Vorpil...
So... yeah, Felguard is nice, but I'd wait until 58-60.