My favorite is when I use H-Delay and I have like 3-4 different delays set up on Aux tracks - so it's just constantly hissing at me. I don't always notice it when I'm doing the mix, but as soon as I reload up my project and get blasted with hiss, I just have to....
I was playing with the H-delay the other day, just seeing what presets are in there (there are some pretty funky ones actually). Anyway eventually I got back to mixing and manipulated the delay to do what I wanted it to do, but couldnt figure out for the life of me why a certain slightly louder word on the vocal track was clipping and distorting like crazy. Checked the pre-fade meters, no clipping, checked each plugin, no clipping, checked all busses, master faders, no clipping, checked monitoring chain, no clipping.
Eventually narrowed it down to the fact that one of the presets I had played with flicked the analog button to number 3, which for some reason clips audio viciously well below 0dBFS.
But yes, I find the hiss annoying - I make my own presets as well. Same thing for compressors, I hate opening them up when they are already doing gain reduction, so I set the default threshold to be high, makeup gain at +/-0.
I leave it on when it's the SSL plugs, off on the API plugs cause I find the hiss too much.
Interesting - if you watch the Waves tutorial on these plugs on waves.com the guy (Barry Wood, author of "Mixing by the Bundle") says the only plugins he leaves the analog turned "ON" for are the API - that they are the only ones that actually model the analog behavior and don't just add hiss!