I've got one of each. Had already bought the Mini12 as a gift for my wife. I was also buying a Studio 15 for my daughter. As she was configuring it, she mentioned that Dell was offering a Mini9 as a special buy-with deal at a steep discount. I hadn't really wanted one. Frankly, the last thing I needed was another laptop. My daily driver is an XPS M1330, and I've got several other laptops all the way back to a Latitude D300XT (build date: May '99) that still churns along fine. But when it worked out that the Mini9 would cost me all of $182 .... what the heck could I do but buy it?
Obviously, at that price, I didn't get all the bells and whistles -- only 8GB drive, no camera -- but I stripped out most of the bloatware, upgraded to 2MB RAM, stuck in a spare a 4GB memory card, and then loaded Portable Firefox and several other programs onto a 2GB flash stick. Even with Zone Alarm and IE7 installed, it's using less than half (3.54 GB) of the HD.
So I'm not sorry I bought it. It's really become quite handy to carry around from room to room.
Only thing I would change is the shiny lid. What is Dell's obsession with shiny lids? Same with my XPS. Has not ONE designer ever used one of these? The lids look awful within minutes after cleaning. Must have been a design decision by a marketing type.