Here's my take on it: I have a black Mk.2 XR2 which is an ex-concours car (won Ford Fair twice and XROC National Day etc.) and when I was regularly showing it I had all the underside painted gloss black and every area detailed as is normal for a full concours car. After I stopped showing it in about 2005 I decided to do the entire floorpan and arches in black underbody seal as I knew I'd never be doing concours with it again and would mean I could drive it more normally without worrying and being a nervous wreck everytime it went out.

Done carefully and neatly it can still look very clean and tidy underneath, it can still be washed down and cleaned but gives you the piece of mind that everything's protected. I used 'Underbody Seal with added waxoyl' and it dries to a nice satin black finish and although it does remain very slightly soft (to allow it to self heal itself) as I said, it can still be washed without fetching any of it off (which regular waxoyl can't).

If you're anything like me, if you start doing the 16i's floor in the grey, like Lee says, you won't use it, no matter how much you think you will right now and every trip out will be a gamble with the weather and it will ruin your enjoyment of the car.