I had an 18v DeWalt circular saw, as part of a kit, and found it to be OK for basic carpentry tasks, but not great at all for cabinet-grade or better woodworking. The blade size limits the availability of quality blades, and this, combined with the low RPMs, just leaves way too much tear-out. The nice thing, though, and this is true of most cordless circs, is that the blade is on the left, making it easier (for me, at least) to see and control the cut.
I upgraded to a 15 amp corded Porter-Cable sidewinder that I'd read good reviews on, it also had a left-side blade, and I could do quite good work with it, provided I used an appropriate quality blade and a guide. I loaned it out to a friend, who had it stolen out of the back of his truck, and I really regret taking pity on the guy and not having him replace it for me, because they no longer make it.
I purchased a Festool track saw and it is simply amazing for doing cabinet-grade or better work. The cuts are clean, safe, and precise and can easily equal the table saw's results, but is still not quite as good for repeatability. But, like all Festool products, it is insanely expensive, and works best if you also buy their equally expensive vacuum to use for dust extraction. I love their stuff, but these tools are definitely blue-sky budget territory. DeWalt now has a similar saw, which is nearly as good, but still up there in price.
At this point, I have a table saw, a miter saw, and the Festool track saw, but the one thing I still lack is a general purpose circular saw, and I'm probably going to fill this gap with the venerable worm-drive SkilSaw. I've thought about getting another cordless, but I don't see the increased portability in return for a poorer cut to be worth it, for me and my tasks, at least.
If I could only have one saw, I'd probably choose something like my stolen Porter-Cable sidewinder. You can do framing with it, cut cement blocks with a diamond blade, and do finer woodworking when used with a guide, or some of the aftermarket products that allow it to do a reasonable job as a track saw. Definitely the most bang for the buck.