I've been using the Lenovo s10 for the past two months, tried several different brands of netbooks before i bought it. IMO the Lenovo is the only one that feels like a "real" laptop, all the others were a bit to "plasticly" and felt like toys. Also, the Lenovo is very easy to upgrade - you just unscrew to screws on the bottom and use a small access hatch to access the hdd and the ram.
Besides the build quality of the Lenovo, the support for PC express card were also a factor in my decision, I plan to add a nice sound card for it in the near future (perhaps the Echo Digital offering). The on board ALC269 is fine, no hiss or bad noise while using the on board headphone out port.
I've upgraded my Lenovo with a 500 gb HDD and 2 gb RAM, the computer came with a factory 6-cell battery (7+ hours of up time). I'm using it daily while away from my desktop PC and while on business trips. Wouldn't leave my home without it!