A good ebony fretboard will feel very hard and smooth, almost like a soft stone. It's absolutely fantastic IMHO....it feels wonderful to play on.
But it really doesn't make or break the guitar...a nice guitar will be a nice guitar regardless of the fretboard wood. Even issues like sustain and tone are dependent on too many variables to say that the fretboard is responsible for a specific tone or length of sustain. The only way to make a decision is to go down to the music store and play a couple different kinds of woods and see which you prefer.
I'd suggest playing a couple Les Pauls. Play a standard which will have a rosewood fretboard, and then play a Custom which is a similar beast, but has an ebony fretboard. You'll either notice the difference right away or it won't be a big deal.
If I was choosing between two identical guitars at similar prices, and the only real difference was an ebony fretboard, I'd go with the ebony. If you're comparing two completely different guitars, then chances are the fretboard won't be the deciding factor in your purchase.
Slackmaster 2000