Worth it.
I bought a Verizon MiFi 2200, so I apparently think that $35/month is not too much. I use it at least a few times per week, and it works better than the "free" Starbucks WiFi... so I'm now saving more than $35/mo on coffee.
I have unlimited data on my AT&T Blackberry, and I can Bluetooth tether, but that's not legitimate, so I don't.
A friend recently got a Droid, and turns on the hotspot when she goes on the road, leaving it off most of the time.