Works OK on a full fat tablet, although it is like using one of those old "slablet" things that used to have a real keyboard and a few lines of LED or LCD display above. You end up with your tablet screen being mostly keyboard with a bit of screen visible above. If you're really lucky and the software writer doesn't have his thumb up his arse, it may even be the bit you're trying to type in.
On a phone screen it's rather less successful. Typing is out, "point and peck" too often results in hitting the wrong character and you really need something like Swype to make it usable.
As things stand right now, if you want to type more than a few lines (phone) or a couple of paragraphs (tablet) or, god-forbid, you need to be able to see the full format of what you're typing, a full-fat computer with a keyboard is an infinitely better option.
Having said all that, I find that the on screen keyboard with Swype on my Xperia Arc S is eminently usable for texts and short mails of a few lines.