I have 2 Jitterbugs in my system. Slight improvement in sound. Very slight. Regen gives greater improvement.
I ordered one from Music Direct to use with well-broken in AQ Dragonfly and headphones (Senn 600s driven by Asgard 2) from a Macbook Air. After almost daily use for at least a couple hours a day for about a month, I could not hear a benefit so returned I it. There was another thread about Jitterbug (started by Ozzy, maybe) and folks there reported hearing a benefit.
I just added a REGEN and two Jitterbugs, one each in the powered and unpowered USB outputs from my Antipodes DX. From the Jitterbug in the unpowered output, I run a Totaldac USB cable, then an ifi purifier/filter, then the Uptone Audio REGEN, then Uptone's short solid connector into my Metrum Pavane DAC. I do not run a cable from the DX's powered USB output, just the Jitterbug.
The REGEN made things sound more natural and relaxed, with maybe a touch of additional tonal density. I noticed less improvement when adding the Jitterbugs but haven't performed A-B comparisons between them and the REGEN. Everything together sounds as good as I have heard from my digital set-up.
In contrast to Philjolet's experience, I tried a Jitterbug in an unused USB input on my Lenovo XP laptop. The Jitterbug worked fine from the start and firmed the bass while improving the focus and image density of the instruments. Now I always go "Bugged"!
That being said, my friend bought one and it worked on a mutual friend's computer but not on his computer. And a different Jitterbug that the same mutual friend loaned him worked fine--so they traded JitterBugs and it solved the problem. I can't explain it, but that's what happened.