That is what it really comes down to. More than the optical performance, build quality, or outright cost (all of which can vary wildly) of any vintage aftermarket lens, it is your level of expectation that should determine whether you bother with them at all. If you are expecting OEM-grade (Original Equipment Manufacturer) performance and build quality out of something that was originally half-or-less of the cost, you are going to be disappointed. It is as simple as that. However, if you reasonably expect (oh come on, since when does that apply on the Interwebs ;-)) somewhat-lesser-but-still-capable performance from that $10 or $20 flea-market find, you may find yourself completely satisfied with a third-party alternative.
Well now. That took care of that in short order. Cheerio...
Okay...okay...okay. You know I can't leave well-enough alone. Read on if you feel the need for a long-winded, over-analyzed version of the above. Otherwise, just go out and keep snapping away with any cheap old light-sucker you find. After all, ignorance is bliss... ;-)
Suffers from a quarter-century and counting film and manual focus SLR addiction. Has recently expanded into 1980's AF point and shoots, and (gack!) '90s SLRs. He even mixes in some digital. Definitely a sick man.