Pepsigirl, it's really quite simple. The presence of a blowjob is much better than the absence of one about 99.9% of the time. I, personally, have given CONSTRUCTIVE criticism on occasion, but only because I thought that it was desperately needed(teeth, WAY too much enthusiasm, etc.).
But, for the most part, I usually keep my trap shut as I am most grateful for the effort, even if it is not the best. For me, it's the thought that counts.

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If the cops didn't see it, I didn't do it!

Old age and treachery WILL overcome youth and enthusiasm.