He has really good sustain and is nice in that he doesn't need to hearth and fountain as much as other chars, but very little burst. So you'll end up doing a ton of plink-and-plunk damage over the course of the match, but he can be hard to "finish" with sometimes. Still, especially as specialists go, he's pretty easy to pick up, can siege well, and can be a good facet of a teamfight even if he can't 1-vs-1 kill much. Just watch your mana.
Also, remember you can "uncast" your zombie wall if you miss by hitting it again if your missed wall is either (a) in then way of your team's pursuit of a fleeing opponent, or (b) you've accidentally trapped a team-mate.
Once you get really comfortable with Nazeebo the real fun begins with using zombie wall in unexpected ways as a little mobile wall; cutting off chokepoints for a few seconds, keeping an opposing team off a capture point, etc. Same way you can see Leoric players use Entomb in some very, very clever ways if they can think fast enough.
