Last week I mentioned in the comments to my Install issue with CTP3 blog that I was using NTFS compression to keep the size of those huge virtual machines down. My experience was that despite of what we hear about compression, the performance of virtual machines is actually better when the VHD file is compressed, as long as your CPU can keep up with decompressing on the fly. Since then Adam and Robert confirmed that they had similar experiences. So it would appear that at least with laptops where disk I/O is more likely to be the bottleneck than the processor, you could benefit from compressing. But I guess even if you didn't, it's still nice to save a few gigs of space.
I just got an external USB2 hard drive and I am planning to do some benchmarks to compare the speed of my virtual machines there with the speed on the laptop disk. Some posts on the Internet indicate that it might perform better because the guest OS would be on a different spindle. If that's the case, I'll probably start lugging the disk around with me, even though it will be a PITA since I constantly move my laptop between the office, Vermont home and Montreal home. But having 250GB of space is still very appealing, especially since it would allow me to build a virtual SQL Server cluster.