Yes, you can upgrade the ram, harddrive and optical drive yourself. There are several websites that show how to do it. I've done it twice, it is really no big deal. Flip the mini over, use the right size putty knife(s) and carefully lift the lid off...from there it's a matter of removing some screws, and then reversing the procedure.
I returned my mini 2.53 because I decided I really wanted to stick with laptop for this year. But for the 2 weeks I had it, I had Logic on it. Used a USB interface, and had some Firewire drives hooked up to it. It was excellent. It's so small that it can sit right on the desk where you can get at everything.
I left the 5400 drive in it, and had zero problems. It's a rare day when someone maxes out their harddrive speed. If you check your meters in Logic, the cpu has always been maxed long, long before the HD I/O has been. If you are running 96K 30+ track creations, a mini isn't even going to be on your radar. Yet, many people use nothing but a Macbook Pro to do some very complicated tracks...and a mini is really just a macbook pro without screen/keyboard/trackpad and jammed in a small box.
I think they are fantastic values.