As noted in an earlier blog post (here) I have a healthy degree of scepticism that these devices will have a long-term future, not least because the growth of embedded modems will eventually render peripheral devices (including dongles) redundant. That said, I have been very impressed with the Novatel MiFi device I've been using. Set up is easy and being able to put it anywhere means I can find a reasonable connection in my Faraday-cage-like apartment. Part of the improvement in the service (relative to my 3 dongle) could be down to better coverage from Vodafone.
Nevertheless I still suspect that the WiFi router will see only modest uptake. Three-quarters of users of mobile broadband (as indicated in the recent Ofcom Comms Market Review) use it as a complement to DSL/cable, rather than a replacement. As a complement it doesn't really make that much sense since for out-and-about use, it's unlikely that most users will need to share. The router is aimed squarely at the replacement market and then only if the owner lives alone or never moves it. There's nothing like taking the family broadband connection away on a business trip to really annoy a spouse/the kids. So there's a healthy opportunity there, but not massive.
For forecasts of mobile broadband adoption by device type, see Analysys Mason's recent report Mobile Broadband Devices: From USB Modems to Where?