Had exactly the same problem as you - my Dell 8200 is great, but far too heavy. Got the X1 a couple of weeks ago. Only taken it on a couple of short-hauls so far - its real test will come next week on a RTW trip.

Thoughts so far. On the plus side:

2.5 lbs with the standard battery. Only 3 hours but it's usually plugged in so no real problem. I have the larger battery as well for emergencies.
The adaptor brick is about as light and as small as my Nokia mobile! The brick for the 8200 weighs a ton.
Great looking (bright) screen.
Great Wifi reception - better that my 8200 with add-on Wifi card or my Ipaq.
Built-in SD and CF slots. Useful for my digi photography.
No fan, so deathly quiet.

And on the negative:

Only a 1.1GHz CPU, so you'll not be playing many 3D games on it if that's your wont. I have put Tiger Woods PGA 2006 on the HD and it seems to cope with that OK though. If you need a bit more speed, add more memory (I have 1280 MB)
Slow HDD.
External DVD (& floppy if you need one). Not exactly heavy, but certainly another thing you'll need to lug if you're going to watch movies for instance. I should add that I haven't tried performance with movies yet either.
No PC card slot. Important to some, but not me personally.
Expense.

All-in-all, suits me perfectly for email, web and MS Office, but it won't exactly set the world alight, speed wise, if you need to run more demanding applications. My shoulder is already thanking me!