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.