What's your opinion on the PC-based ones, I mean the inexpensive kind, not the multi-thousand dollar rigs
To be honest, I've never tried them; there are some schools of thought that say a scope should be a separate standalone instrument, but most people have a computer on their bench nowadays anyhow...
I have heard a lot of good things about the BitScope (http://www.bitscope.com/) - its fairly inexpensive, has a lot of features, works well with Linux, and to an extent is even "open source" (well, they have the design documents and files for the original bitscope that was published in the past). If I were to buy a PC-based scope, I would lean toward one of those.
There are some PC-based scopes on Ebay and elsewhere, some seem "ok", but they don't have the sampling rates or bandwidth (or other options like the logic analyser) that the BitScope does. Some aren't much better than the handheld DSO already posted.
I don't have a scope, and even have been thinking about checking the forums and google for any particularly good microcontroller-based projects. Seems a natural, so I have always assumed there would be a thousand poor implementations, but few halfway decent ones.
Yes, there are a few "halfway-decent" homebrew scopes out there (both PC based and standalone, actually). If you have the parts, and want to try them out, they are certainly worth looking into. Part of the thing to keep in mind is what you want to sample, what you expect your bandwidth needs to be, and what you think they might be in the future. Many of these low-cost, low-bandwidth scopes are perfectly OK for low data-rate applications (checking IR remote outputs, audio signals, servo control signals and the like); but if you intend to attempt to listen in on higher-speed signals (higher-speed RS-232, SPI/I2C, USB, etc) - or you need more data lines to sample (where a logic analyser might be a better deal), then many of these homebrew scopes won't address that.
That being said, I've even been tossing around the idea of a basic logic analyzer/oscilloscope design that uses Arduino (easy enough, plenty of inputs, fairly fast) with the TVout library. I don't know about others, but that lib provides a very low cost, high-complexity display option that most of us have already.. many of my current projects use TVout because portable NTSC displays are readily had from broken portable dvd players, if not the good ole family tv. I would love to be able to flash the boarduino with software making it a logic analyzer/scope when I needed it, and then reflash the chip for other purposes after I was done with the need. No need to dedicate hardware, except for the amplifier circuitry and such. Given the settle time of the a/d and all that, I suppose it might not be fast enough for fast signals, but audio and logic ranges ought to be well within sampling range.
I say "go for it" - its a project I've thought about myself (too many other things in the way right now, though) - although I was thinking about doing it with a GLCD instead of the TVOut - but either should be OK. You're right that the data rate isn't going to be very fast; your bandwidth is going to be very low (you won't even be able to get much audio bandwidth, actually!) - but for many purposes it may work out just fine.
There's also this kit:
http://www.seeedstudio.com/depot/digital-storage-oscilloscope-with-panels-p-514.html?cPath=104_108
Another option, if your only needing logic level signals or a logic analyser:
http://dangerousprototypes.com/docs/Open_Bench_Logic_Sniffer
That's another thing - if you care more about the logic-level transitions, rather than waveform shape (or square-wave waveforms), then a logic analyser might be a good deal. Actually, when you need it - having both (and it can even be handy as a single tool, like the BitScope - which is something I like about it) can be something useful...
