Great question and I just happen to have a Virgin Mobile mifi 2200 wireless hotspot.
I went into network settings, selected wireless networks and the Roku picked up the Virgin Mobile Mifi hotspot. I entered my password and it connected me. So far so good.
I went to Netflix and it loaded, not much longer than my usual load time using a Wireless N 5 ghz connection.
Selected a movie in HD, Cheech and Chong's "Up In Smoke". Selected play. Movie loaded, gave me HD quality as it loaded, took about two minutes to fully download (retrieving screen). Movie started and a few seconds in reloaded/rebuffered, this time Netflix quality showed 2 dots. Watching movie as I type this, about 20 minutes in and no reloads since the first which took picture quality from HD to 2 dots however the quality is very good. Not quite HD as usual but well above a SD picture. Picture is not as sharp as HD but much better than I anticipated with the mobile hotspot. Very watchable.
As to cost as one poster mentioned, not an issue. The Virgin Mobile Mifi hotspot is $40.00 a month with unlimited usage, uses the Sprint 3G network (Sprint owns Virgin Mobile).
I can say from trying it this morning that the VM Mifi 2200 will work with Roku. Will try other channels later.
55" Insignia-Roku 4K TV, 40" Hisense Roku TV, Roku2 XS #13A164000326, R3 #1GJ36A038993, R4 wired to Netgear WND 3700 router, 300 mb TWC broadband (Subscriber- 2017 MLB.TV Premium, 2017-18 NHL.TV, Hulu+, Netflix, Amazon Prime)