We have the Wilson Mobile 3G and it has worked very well for us over the last several years. We use it primarily to access the internet via our tablet (which gives us local wifi for our computers). But it has occasionally been useful with the cellphone as well, but cumbersome.
As already pointed out, one big advantage is being able to place the antenna higher and use a directional antenna if you want. Up north I have a separate antenna high on the roof of the two storey cottage, no directional antenna is needed.
With our trailer I use the same amplifier but have two additional antennas one directional and one non-directional antenna that is often good enough. I mount the directional antenna to long Al tubing (those who have used snow roof rakes will know where that came from) and strap it to the trailer's ladder.