Check out my screencast analyzing the cost and functionality of this product.
Limited mobility (since it's attached to the wall, this may be an understatement), EXPENSIVE, a little dated.
After the initial cost of the iPad (pick your model, Apple Care plan, etc - mine cost about $600-$650 after all was said and done) I added $30 worth of apps (Air Display and Ink2Go - although Ink2Go is optional and the more expensive of the two) and have a fully functioning mobile tablet that will project through my computer/overhead projector onto my screen at the front of the room. I can attach my second iPad and make the screen even more accessible. I can annotate, run iTunes, manipulate documents, windows, Tweetdeck, etc. One suggestion online was to purchase a stylus ($10) because you'll be able to manipulate that better when it comes to writing and highlighting.
I can also do extended display. Instead of mirroring my laptop, I can set the iPad to become a second extended display and open Safari windows with research on one and have a Pages/Word document up on the other. Or move tool bars to the iPad and fill my laptop screen with the document. It doubles your screen capacity and works in landscape or portrait. The Air Display program works with both PCs and Macs (there are glitches with Lion at this point, however - I haven't updated yet).
Finally, in addition to the mobility and cost that make the iPad the best option of the three, there's the best factor of all: YOU GET AN IPAD.
We all know about the hundreds of "apps for educators" that exist, for a few dollars (or even for free) you can expand the educational capacity of the iPad exponentially more than could ever be done with a SmartBoard or Mobi.