Pros: Fast but still portable, lots of options. EVDO capability and D-series compatibility
Cons: Higher-res LCD would be nice, battery life could be better.
I owned a D620 for about a year (see my review here), and following that I purchased a D630 to replace it. I won't go into great depth, as they're very similar notebooks, but I encourage you to check my D620 review, as I'm going to go over mostly differences.
At it's core, there is little different between the D620 and D630. The D630 introduces the new Santa Rosa chipset, more formally titled the 965 chipset. What does it offer? Faster, less power, and all the usual stuff that comes along with new chipsets. The RAM speed is still capped at DDR2-667, although along with the new chipset comes the next line of Intel Core 2 Duo CPUs, which offer a variety of speeds and an increased 800mhz front side bus, up from 667mhz.
In terms of graphics, the offerings are up from a GMA 950 to GMA X3100 graphics on the integrated side, or if you'd prefer a discrete offering you'll have a nVidia Quadro NVS 135M up from a Quadro NVS 110M. What does this mean in terms of graphics performance? The X3100 is a good improvement over the 950, although it's not a huge difference. Vista's Aero interface will run smoother, and you'll see an edge in gaming performance, but at the end of the day it's still an integrated GPU. The Quadro part isn't much of an improvement, but overall the performance of the dedicated video will still beat out that of the integrated part.
The best part about the D-series is that it's accessories are all still working here. If you've got any D-bay device, or a D-Dock or port replicator, keep ahold of them because they'll still work here. Similarly, the batteries from your D620 notebook will still work, so if you happened to buy a new one to replace a fried D620 battery (I know I've seen a lot of people do that), you'll have a spare for your new notebook!
The only real marked difference in construction is the addition of a 4pin 1394 (Firewire) port, and the removal of the nearly useless IR port. Don't get me wrong, IR has it's purpose, but on modern notebooks it's all but useless.
In terms of performance, you're not going to see a big difference, although it is there. There are more advanced options in terms of processors, although you'll pay a little more for the better performance. Battery life is about the same if you take a similar configuration, although if I had to give the edge to one of the two, it would probably be the D630. All in all though, it's still a VERY capable notebook which has some excellent options for mobility if you're not looking for a very large notebook.