Well, I kind of think that if the point of building the reprap is to make parts for the reprap, then it's kind of a waste.
That said, if you buy a cheap kit, you'll end up doing exactly that for a while. As I type, my i3 is making a second set of feet. The ones I got were crappy, so I modified the original design to something that suits me better, make a set, then put them on the printer. Now I'm making set #2 in case something happens to these. Of course a spare set of everything else, too, though I'm going to redesign the X axis parts because mine is hacked together with zipties.
So far, I've been mainly playing with things, but I've made some small things which are kind of handy to have around. A curtain rod hangar to replace one that broke, and I can't find a replacement to buy in the same style. It has the print lines on it, but you can't see that from a distance at all. I replaced the bezel on a small 7" LCD that I plan to use in the car with one that will fit in my dashboard. I made a couple of gauge pods for my car as well, and will also be making some speaker mounts to allow me to use nicer speakers in a nicer location. Car parts are mock-ups for now, though if I re-print them in ABS, I could probably use them that way. I don't really trust PLA since the car can get awfully hot in the sun, though it ought to survive, I'd feel better with more leeway, knowing that it's unlikely to warp anything. If I choose not to use ABS, I will use the PLA parts as a plug to mold them in fibreglass or carbon fibre (Making the molds very smooth, of course.)
I have many other small parts that I want to make around the car audio/video/security systems, holders for things, separators for wire looms, and the like. With a 3D printer, I can make button and switch plates that use the factory locations, as well as house displays for the aftermarket engine management systems, gauge pods and the like. So basically, it's a hobby that will help with other hobbies.
I also make a lot of DIY audio equipment, I've made several home audio amplifiers, signal processors, equalisers, and the like. Currently most of them are mounted on stand-offs and screwed down to pieces of wood. I'd like to make some nice cases for these pieces. Though many are too large to 3D print on the reprap, I could make accents, corner pieces, and knob panels on the printer, then use flat stock for the main components. I could also 3D print holders that the circuit boards can be screwed down onto. I also have made a few DIY video projectors, and there are a few things that I have hacked together that could use a purpose-designed part instead.
Other projects planned are modded computer cases, speaker box parts, maybe a kitchen knife holder...