I wish I had found this thread earlier. I too have been "around the block" with those cheap-o lenses marketed by eBays sellers, by Adorama, etc. For the worst of 'em, with shipping, I shelled out $70+ and later, had to eat the $18 initial 'shipping/handling fee', the $8 return shipping cost, plus 20% restock fee. So, those buggers get to re-auction the junk, endlessly scamming $20-$22 per "round trip" as each new chump comes along.
Crystal Optics == Digital Concepts == they're sold under various brands and in diversely different packaging. And without exception, in my experience, they were all junk.
After getting burned repeatedly by chasing "no name" brands, I moved onward... to getting "burned" when buying several brand-name lenses (yeah, from eBay sellers). The $40+shipping pair of Kenko lenses wide/tele (new, in box) turned out to be leftover/unsold stock "camcorder" lenses. Ah, a camcorder only needs to resolve 480 lines of resolution vertically... so, brand name or not, I discovered the manufacturers don't "overbuild" lenses for camcorders using low-dispersion glass. Again, for macro use -- junk.
The sole "generic v/s name-brand" scenario in which I found the name brands did NOT excel has been diopter close-up lenses. Wondering how many, and in what combinations, I could successfully stack diopters... I bought a "drawerful" of them across several months. Not only did a given Hoya +4 not match the magnification of a Vivitar +4 ...three different (identically stamped) Hoyas yielded three different degrees of magnification! So much for quality control. At some point while researching I read (and fully believe) that there are/were only ever a handful of plants producing these "close-up" diopters -- they brand 'em to meet the specs of the purchaser, using the same ol' same ol' glass regardless.
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Anyhow, most of my faves among the diopters turned out to be the generics, bot for individual use and for stacking. Along the way, I had purchased a boxlot of 12 (new, in boxes) +10 diopters, curious to see how many I could stack, and determined to find out first-hand whether "more pieces of glass between you and the subject always leads to a degraded image". Turns out I could stack FOUR +10 diopters and get a satisfactory image for really small subjects (grains of NaCl, how boring) but the CA, and the teeeeeeeny working distance render the stack useless for larger subjects. All in all, *no*, regardlesss which of myriad combinations I tried, and which brands used... there wasn't a noticeable difference between images shot using a +4+3+2+1 four element stack vs using a +10.
When stacking lenses, though, working distance -wise you're at the mercy of the front lens' specs.
So, I began shopping for used SLR lenses to used 'reversed'. Hooboy! Talk about ridiculously small working distances and maddenly shallow DOF! Yeah, reversed lenses are LOTS of fun, lemme tell ya...
...and since a reversed 50mm works "good" (within the limits of managing shots with respect to the working distance)... a 28mm (f2.0) lens would work even "gooder", right? Ouch! No, not at all. With either of these (and a few other lenses I bought and tried to use "reversed") the lenses produced miserable results at apertures larger than f3.5, like you're shooting through milk glass (at a loss how better to describe it).
After all that messing around, I found that for most of my macro shots (insects, typically -- inert specimens retrieved from window sills) my "weapon of choice" is stacked diopters. For shooting insects in the field, again my choice is diopters (Raynox DCR250 would be preferable *IF* its working distance wasn't so small) and the rest of the time, I really don't need to "bother" trying to achieve a tigher field of view than my A620 (or A75, or whatever) lens natively provides in macro focus mode.
There's still one Raynox lens (and a Raynox kit, of 4 lenses) I'd like to get my hands on to test, but... except for crystallography, using cross-polarized lighting, I can't imagine taking an interest in photograhing details of any teeeensy-small subjects.
