I've been tethering my n810 over the $25 Unlimited plan for a few months. No complaints.

Speed tests show about 20-25Kbytes down, 12kbytes or so up.

I'm told the difference between the 25 plan and the 9.99 is that the 25 has zero port restrictions, ie IM voice p2p https ssh etc work. The other is run through a bandwidth/port restriction limiting proxy.

I'm currently using google voice to make all my calls with my GV number set as a MyFaves (meaning unlimited minutes)...so for the cost of a data plan (I share a family plan with others) I get unlimited minutes, texts, and data!