I started out using tun, but switched to tap since I didn't like the use of a /30 subnet for each PC (I need to support Windows). I found that to be wasteful and confusing.
Then I discovered the "topology subnet" option on the server. Works with the 2.1 RCs (not 2.0), but it gives me all the advantages of tun (no bridging, performance, routing, etc) with the convenience of one (sequential) IP address per (windows) machine.