I've been using the AvPad from http://www.picsupplies.com as my iPad kneeboard, and I love it so far. I've been using ForeFlight since it was first released and it works great. They will be adding capability for XM Weather to it later this spring. The fuel price map is great to help with planning longer cross country flights and finding cheaper fuel at other airports along your route. Even though I have a 3G version, I also use a GlobalNavSource.com Dual WAAS bluetooth GPS that I like better then the other two that Sporty's is selling. It works great and has a rubber mat that it fits in so it won't slide across the glareshield.
As for the cost of the units, remember that (for new devices) you pay $130 more for the 3G version. All of the external GPSs are around $100, so you can save a little bit by buying just the WiFi version of the iPad and an external GPS. Just be sure you don't have a *need* for 3G data.