I just installed a slab of travertine in my master bath replacing tile/grout. It is beautiful, but I cannot speak to durability. It has been sealed, and I will seal it regularly. If you are using travertine tile on your floor, I suggest getting it delivered, then take several tiles to match the choose the slab.

I adore travertine. I have it (tumbled) on my kitchen backsplash and (honed) in a seldom used front entry. As much as I love it, I chose to do a travertine look porcelain tile for my bathroom floor and am using the real thing on the walls.
I personally would be very nervous about using it on the bathroom (and kitchen) counter. I sealed the heck out of the kitchen backsplash and nothing that has splattered there has stained. But with all that a bathroom counter endures...I don't know. I'm afraid I'd just be too worried to enjoy it. I have some sort of stain on my honed entry floor. It got there when workers were in and out during remodel. Someone kicked up the brown paper that was protecting it :( I haven't tried a poltice yet but a natural stone cleaner hasn't removed it. The floor was sealed with a water based natural stone sealer.
Monica

Trudymom, we've been in just over 2yrs and have limestone slabs in four bathrooms. No problems. They are sealed (but not enhanced). I have lots of travertine flooring (bathrooms, pantry, laundry, kitchen, etc) also sealed and havent' had problems with either. I would do it again in a heartbeat.

I've had a travertine slab counter top for over two years. I'm the main user. I haven't had any problems with it, but I avoid using vinegar or other acidic products anywhere in the bathroom. I'm also very careful with hair coloring or nail polish remover. But I'd do the same with any material.

It's so beautiful.