I agree that the Dynojet kits may not be the "end all" product for every circumstance, but they seem to take a lot of guess work out of the equation and get you at least in the ballpark for about $100. You may not be out $100 by trying different jetting/needle configurations on your own (although you might), but the time involved in pulling/reinstalling the carbs (if you keep the airbox) on some models will quickly make $100 seem like a bargain if it saves you two to five carb removals/installations.
My own experience with Dynojet kits is fairly limited (three bikes, all GSes with CV carbs), but I'm happy with the results in each case. All of my bikes have the stock airbox, stock exhausts, and K&N OEM-style replacement elements.
On the 700, I purchased the combined Stage I/III kit that normally sells for about $110. I got one off ebay for $40.

I installed the Stage I stuff and followed the directions verbatim. Bear in mind that I've owned this bike for over 20 years, and it was entirely stock the whole time. What I immediately noticed was:
- warm up time on a chilly morning went from 3-5 minutes to less than a minute
- the midrange stumble from 4K-6K rpms was gone. Power is now much more linear from 3K through redline
- the plugs went from grey (a bit lean) to the perfect toasted marshmallow beige color on plug chops at all throttle positions
- mileage during general riding dropped from about 43-46 mpg to 40-43 mpg
Overall, I couldn't be happier with the results.
My '81 750E and my wife's 550L both became happier as well with the Stage I kits. The 750's warm up time also dropped drastically, and it's mid range improved noticeably. I didn't have enough experience with the 550 to really get all the details on it's operating performance (I installed a kit as soon as I had it running), but I can say that it takes a lot less choke to fire up, and idles well within a minute or two, not five to ten minutes like it needed before (the cv-equipped 550s were notoriously lean, and even borderline unrideable in '82, their last year of production).
Stage III kits with pods and such I can't report on firsthand, but I've talked to many people who have used them, and generally the consensus is that they are either spot on, or close enough to save you a lot of time.
Your mileage may vary, of course....