I bit the bullet and bought an Overdrive last night at Best Buy. Right now they have it for $50, or free if purchased with a WiFi device. The salesman gave me the free price even though it was all I was purchasing - I didn't even ask! YMMV.
First, as every review of this makes clear, your experience will depend on where you live. I am in Austin, TX, and the Clear/Sprint 4G service here is reasonably mature and well built out. At my home in the north central part of town, I was getting what the unit described as a "20% strength" connection, but a speed test revealed downloads over 4 Mbps.
My office is in between the UT campus and downtown, and although I have a window, I am completely unable to conect to 4G so far. I bought the Overdrive primarily for office use, so that's a strike for me. I do get 100% signal strength over 3G (I think we have a repeater on our building).
Still, for $60/mo, I get 5 GB of 3G coverage a month, plus unlimited 4G when I can access it. A MiFi from Sprint or Verizon would only give me the 3G part for the same price.
Many users have complained about the unit's construction. I've read about overheating, lock-ups, dropped connections, etc. I haven't experienced any of those yet. Supposedly firmware fixes are in the works - we'll see.
One nice thing - Sprint now has a 30 day money back guarantee. If it fails or bugs out, then I can take it back, get my money back (or could if I paid for it), and get the ETF waived.