I played one for a couple sessions around level 4 or 5, but ended up dissapointed. In combat, he was fairly useless because flanking doesn't help you with Sudden Strike, and because of that and lack of the rogue's uncanny dodge, melee probably isnt your best option. I was planning for Shuriken throwing with Master Thrower -in order to actually use Sudden Strike more than once(maybe twice) per combat since Ghost Step is pretty limited in its use- but he was just frustrating to play. Now that PHB2 is out with the "deadeye shot" feat, it might be slightly more viable, tho it's still 6th level before he qualifies.
The other frustrating part for me was the skills. It's got a skill list nearly equal to a rogue, but fewer skill points to spreat around, so you're gonna end up short somewhere.
But I think I would have enjoyed it more if I'd have gotten to use some of the skill abilities more too. The Great leap ability, superior acrobatics, and eventually Speed climb would have been fun to use. It's a cool class, but it's probably more enjoyable at higher levels.