I purchased the Incredible but did not have a good experience. After 25 days I finally decided to return it. In between the time I returned it and purchasing the EVO I loaded MyWi on the iPhone for my iPad. It worked pretty well although drained the battery pretty quick.
I was skeptical about the EVO. The incredible felt cheap and I had all sorts of bugs in the OS (lots of crashing, apps eating battery, etc). I didn't pre-order. However, I walked into a Radio Shack this afternoon and they had a few so I purchased one. The hotspot feature is quite usable. I've used it on both 4G and 3G... To my surprise it is actually really fast even on 3G. In fact, I don't notice a huge different between the two. Latency is something you must contend with on the mobile network (the time it takes from clicking 'go' to the time it starts to render the page). However, I'd almost argue it is as snappy as my cable modem and here in Austin, TX we have excellent home internet service.
SO..
The long answer to your question: I've been pleasantly surprised at the performance. WiMax (the technology) will get limited coverage inside buildings. Unless you're close enough to get cancer from the antenna don't expect too much. But, again, I'm quite impressed with Sprints 3G coming from an iPhone 3GS (AT&T).
Hope that helps!