So I just got mine and boy is it nice. I previously used a 2.5'' thick 15.6'' desktop replacement laptop from 2009, so the size is ... a shock for me. Also, the fan hasn't turned on yet, compared to the constant hissing of the old machine ... I now hear nothing but the distant growl of the refrigerator in the kitchen.
It comes with 14.04 and I'm running the upgrade to 15.04 as I type.
If there is anything you want to know - just ask.
The XPS 13 probably is one of the best Laptops out there not made by Apple and Dell is going to offer it with Linux out of the box. The M3800 is a much bigger machine with 15.6'', but also has more power and a second drive bay.
This is now the third generation of the XPS that is offered with Ubuntu pre-installed, so it looks like it actually works out for Dell, which is awesome.
Personally, I guess it won't make any difference in Linux-adoption, but I also don't care about that. Also, installing Linux is quite easy, 'tough it's nice that you can be sure that you don't have to iron out kinks. What I like most however is that I don't have to pay for a Windows license anymore.
The XPS 13 DE is out now.