They're a compromise. If you already have a wifi iPad, then they're a good idea. If you're still wondering which iPad to buy, I'd fully consider your options. Mifi (etc) devices can be faster than the 3G iPad's cellular connection (I believe), but their battery life isn't even remotely as long. Mifis also take longer to start up, find a connection and just generally "get working" than the 3G iPad's connection.

Honestly, if the extra money isn't prohibitive, I'd strongly advise getting a 3G iPad. You're going to want the iPad to be the only tech you carry, and overall I'd say a wifi iPad + Mifi is a poor substitute for a 3G iPad. Fraser wrote a blog post on the topic too:

http://speirs.org/blog/2010/5/29/of-3g-ipads-and-mifis.html