Sure, I've done a boatload of DIY stuff. I don't mess with floor pedals much, I prefer rack gear. It's a lot easier to recycle that way. I've had a couple of the same cases for over 20 years, with dozens of different circuits residing in them. Once you have a nice parts stash built up, something like a Big Muff circuit should cost you next to nothing to build. I use the little Radio Shack breakout boards for IC based projects, they're like 2 bucks a piece or so IIRC.
I started with the Craig Anderton circuits, and my favorite bass fuzz is based on one of his. The big Muff is at the very bottom of my list, but lots of people love 'em. Modifying existing pedals is a good way to get your feet wet, certainly.
I recommend a few trips to the library. Get the Anderton books, the Op-Amp Cookbook, and whatever else grabs you. Also look at the Ampage BBS for a lot of links to cool EFX DIY stuff. That's a great resource for tube amp info too.
I've been using my own preamps onstage at "real" gigs for many years. I've had plenty of good players use my stuff, and love it. There's little need to reinvent the wheel, you just have to know where to look to find the right stuff to copy. Then, it's easy enough to add your own touches to the circuit.
If your time is "free" you'll save a fair bit of money, eventually, but don't get into DIY for short term economic reasons, IMO.
luck,
Charlie Escher
PS: If you follow the links in my profile to my website, you can hear a few clips made using DIY preamps and fuzz. I have some stuff on Soundclick too, if you prefer streaming audio: [Invalid or Expired Link Removed]