Well thats weird...cause I
know from a
really reliable source that the Buffering in the M5 is actually really good...BTW...That reliable source, is the M5 that is currently on my board.
I tested both the DSP bypass, and the "true bypass" options by putting the M5 in a true bypass loop. Then with no effects engaged, set it to dsp bypass, and switched back and forth on the looper to see if there was any tone difference. Did the same with the true bypass option on the M5.
Results? The True Bypass signal sounds literally identical to the signal with no M5 in the chain, and the DSP sounds so close its barely noticeable (bout as much difference as a boss pedal makes). So your source is either mistaken, mis/uninformed, or has ears far more magical than me and my bass player (she also heard no difference).
I think what some people hear as "a bad buffer" isn't actually the buffer...All the Mseries pedals have a built in universal noise gate, that is always on...unless you go through the process of turning it off entirely (explained in the manual, its easy)...That noise gate, definitely sucks the life out of your sound, There is a definite difference with/without the gate. So maybe your reliable source, neglected to turn the noise gate off?
That said, I had heard that early runs of the M13 and M9 had buffer issues, and I have heard TONS of stories about how terrible the buffers are on the DL4, MM4, and FL4... Whether thats true or not, I don't know, cause I never personally tried any early M9's. But the M5 is fine, has a good (dsp) buffer, and Has a true bypass option for those that don't like the buffer.
I will say though, the foot switches on ALL of the Mseries pedals, are terrible, and feel cheap (all line 6 pedals really). So they definitely need to be upgraded, if you're planning to use them live. But beyond the switches the M5 is a great pedal...and This is coming from a guy who HATES line6 (seriously HATE everything else they make).
And FYI you can get the switches on any Mseries pedal or any of the DL4, FM4, MM4, etc upgraded to high quality switches, for a very reasonable price. He Also has mods to improve various other things on the Mseries pedals too (again very reasonable prices)...by JHV3
http://jhv3.squarespace.com/m9-mods/