There are different ways to go about that:
1-simple slowing down on timeline, % or time remap.
2-using plugins, like Twixtor, or going to Shake,
3-exporting clips via Compressor to different frame rate,
at higher resolution, DVCPRO HD for exapmle and then edit it in FCP.

My question: if someone actually done compareble testing
FCP directly, is result pretty good? How does it compare tp DV slow motion
and to 2,3 assume it was shot 1080i 50 or 60??

Thanks in advance!

Good question (i dunno). But I'd like to add onto it:
What's the best shutter speed to shoot for, knowing it will be in slo mo?
The higher the better? Or will grain be a problem at some point?

In my latest project I worked with 25p, in a DVCproHD timeline. To work around interlacing in time remap, I did all retiming in Shake, with the new smooth adaptive slowmotion option. Worked fine - and I'm not happy with FCP slowmos in a progressive project!