Looking at the specs alone...

I've not had the chance to get hands-on with the XPS13 myself but I can certainly do some comparisons to existing 13" ultrabooks and here's what I've found;

• Asus UX31E offers higher screen resolution
  
  • It can be limiting to have to spend more time scrolling while working on source code.  The HP Folio 13's resolution is like the XPS13.
• Asus UX31E offers next-higher-model CPUs
  • As much as I wish everything could be multi-threaded this is just not reality yet.  There are times when every bit of extra clock speed can help and I've definitely run into cases before where more clock will get much more done.  I understand the form factor and the model differences aren't significant so this isn't a major point.  The Toshiba Z835 offered the slowest CPU of those compared.
• Toshiba Portege Z835 has 6GB of RAM (exp to 8GB)
  
  • The only one of the models I looked at to exceed the 4GB mark this sets the Toshiba apart from the rest quickly.
• Acer Aspire S3:  240GB SSD
  • Across the board the 13" ultrabooks are all 128GB SSDs until the $1000 barrier is crossed except for the Acer.  The lower model S3 at ~$850 already has stepped up to the larger drive.
• Asus UX31E offers Bluetooth 4.0
  • Another finer point but something fair to note.  May make a difference for some users.  The comparable Folio 13 also sports Bt 3.0 like the XPS13.  The Acer Aspire S3 also offers Bt 4 like the Asus.
• Asus UX31E is larger by about 20mm (D) while slimmer (H)
  
  • For an ultrabook size is as critical as any other feature.  The HP isn't as thin as either the Asus or the Dell but does have the smaller depth compared to the Asus.  The Dell isn't quite as thin as the Asus but is similarly smaller in depth.  The Acer S3 has the footprint of the Asus and the thickness of the HP.
• HP Folio 13 has a larger battery than the Asus UX31E and the XPS13 both
  • When we're talking about a form factor designed to travel the battery is certainly worth some consideration and in this case the Folio is sporting a 59WHr battery compared to the Asus' 50WHr and the XPS13 at 47WHr
• HP Folio 13 and Toshiba Z835 offer an Ethernet jack
  • Unlike both the UX31E and the XPS13 the HP includes an ethernet jack for hard-line connectivity.  This can be valuable in that those wanting gigabit speeds when they're at their desk won't have to sacrifice a USB port to get it.
• Toshiba Z835 is the weight winner at 2.5lbs/1.13kg
  • Asus UX31E and XPS13 matched for weight at 1.3kg while the HP loses out coming in at 1.5kg
• Asthetics
  • XPS13, what I can see, is a very nice looking laptop with the CF look being a nice touch.
  • The Asus UX31E I find to be on par with the XPS13.  It's got a sexy look with all the brushed metal surfaces and super-slim styling.
  • The HP Folio 13 is very plain and boxy which is typical for HP.  For some this might be preferable but I find it rather dull given the class and form factor.

Given the specifications and press shots alone the most attractive are the Toshiba due to it's lighter weight, large battery, and more RAM, the Dell for it's size, styling, and protection, and the Asus' size and screen.

If the XPS13 had the screen and CPU of the Asus with the memory and battery of the Toshiba it would be a far-and-away winning package.  I can appreciate that the memory and battery upgrades could easily add a little size or weight to the package but being one of the only 13" ultrabooks to offer a better screen with a better battery and up to 8gb of RAM would, in my opinion, set it apart enough to justify that extra size or weight.  Laptops only offer limited future upgrades and being able to get a better screen, CPU, and battery initially makes a big difference.

Having not handled any of these specific models this is only what I've gathered looking around online.  I've used Dell, HP, and Asus laptops extensively in the past which does weigh in on what is probably some bias.

Also compared:

Lenovo U300.  This ultrabook matched most of the negative points of all of the other models and combines them into an uninspired package.

Samsung Series 9 NP900X3C.  Similar across the board to the others but with a higher cost.  Only notable features to compare is that they're using 3rd generation Core CPUs and a higher resolution screen like that of the Asus.  Still an i5 rather than an i7 and a 128gb SSD at over $1200.