I learned on a Hobie 18 about one year ago. The only time I have difficulties is when singlehanding. But I had no experience sailing before this. I bought it thinking it was kind of an adult optimist, and kind of just shoved off and sailed.
I don't think and 18 is too much boat to learn to sail on.

Raising the mast is a bit of a hassle though, if you have to set it up every time you want to go sailing. But with crew on board to handle the jib, and pull up the daggerboards when coming in, the boat is a blast!