I have used A Dell prebuilt and would still have it if I hadn't striped it for parts to build my Intel system. It was pretty good for what it was. E-521.
As for laptops I own a Toshiba that lasted from 1995 to about 2 months ago. The drive finally pooped out. I also own 2 HP's. One is a dv6205us, little over a year old and running fine. The other is a recent acquisition. OmniBook 6000 running w2k. Nice little rig slated to be given away as soon as I get another power pack for it.
And yes I owned an old Apple powerbook. The trackball mouse went out so it got sold for parts to a local shop. It was fun to use. Different but a lot of fun.
As for buying prebuilts I'd stick with the major player in prebuilts, Dell. People think they're crap but for the price to power ratio, you could do worse.
But you don't learn any thing about your computer and any changes you want to make, well drive down a back Alabama road after a good rain and you get the idea.
Building your own is not as easy as some make it out. Putting it in the box and adding in the goodies is, but only after you do your homework. Making sure this and that part works with the motherboard. Save your self major league headaches and shop till you drop. Ask questions and get intellegent answers from seasoned builders.
Then get the best bang for the buck and go for it.