Ask a Guitarist

Fretless guitars (and basses in particular) have a distinctly different tone. I'm not too sure how it would sound with a guitar but basses have more 'growl' and a more pizzicato sound to them. Some fretless guitars have markers to indicate where the frets would normally be but you still have to learn to play by ear rather than sight (looking at what you're doing.)

A slight note on the finger pain thing. If it really starts to hurt - stop. Cut down your playing time until it becomes less painful. Also, wash your hands before you play, this way you get less greasy and sticky crap on your strings making playing easier (and making your strings last longer.)
And about not using too much pressure when fretting - again, if you feel pain - stop. Not only does it hurt your fingers, you're also putting more strain on your arm & shoulders and RSI hurts like hell. If you are feeling pain in odd places, check your posture and your left & right hand positions.

How ya doing, buddy?