I've just finished upgrading the hard disc in my 1st gen macbook pro. I have to say you really see just how well built they are when you take them apart.
All the screws are works of art in themselves and everything inside is really neat and tidy. There was no dust inside despite it being well over a year old.
I've been scared to take apart until now as its far to expensive and useful to break! However I decided it was time to stop being a wuss and get on with it. The original hard drive was only 100gb so it was time for an upgrade. I went for a 250gb WD drive.
I'm still reinstalling os x, but so far things are going very well (There's me tempting fate!). I had a few issues getting the bluetooth module out of the way of the old hard disc.
One thing that really surprised me is that the hard disc is not screwed to the chassis in any way. It uses four rubber grommets to hold it in place. Clever thinking from apple that.
If you are putting off an upgrade don't its well worth the effort.
PS: Big shout out to www.ifixit.com for their great guides on taking apple's apart!