I have a Digitech whammy IV and the Boss PS-II. You can't really use them on chord or double / triple stops or it sounds like ass. You can happily take stuff up a couple of half steps but other than that ....blah !

Generally if you are going to use them on clean sounding guitar, IMHO, do the shift quickly as the single will break up !! If you are using a lot of distortion, well, it will sound just fine !