I got tethering a few weeks after launch of 4...i've really enjoyed having it (having been a clearwire mobile internet customer before, this is just better all around). Connection speed is good, and i avoid up/downloading too much while tethered, its mainly just for email and web surfing...
The first month i ended up using about 3gb, so i was charged the extra $10 on top of the data plan...but heres the kicker, the tethering plan for 2gb per month was only $5/month MORE than i was paying for the unlimited plan without tethering ($30/month)...so technically, i got 3GB of data service with tethering for $15 extra per month over my old rate...if im really using it alot, id prob see 4GB of usage max...so you're talking $25 over my previous plan with NO tethering...
In my case, i was paying $30/month for clearwire with spotty service and alot of downtime...not to mention a VERY limited coverage area (even being literally in midtown ATL 90% of the time)...to me, the iPhone tethering has been a good value, and exceptional in usability.