Just expect to put in a lot of effort learning how to play the fretless well. I generally find anyone who says it's "easy" has lousy intonation and tries to force glissandos and slides onto every transition. Playing with real precision is NOT gonna happen overnight, or in a week, or even a month, but it's entirely possible.
As to lines/no lines, I have and use both (1 lined, 3 unlined). I don't see the big deal either way. No lines is GREAT for the ego - but only if you can play well, or you just can't hear how bad you really are. Lines are GREAT for the studio, where excellent intonation is critical. But relying on the lines instead of your ears is disastrous. It's all about the ears - the lines are just a nice guide in some situations. Even Mark Egan and Will Lee and Marcus Miller and many other studio cats use lined fretless basses.
Overall, I LOVE fretless and use it all the time. If you put in the effort, it's really fun and rewarding, and not limiting at all. If you're impatient, then it's probably not for you.