I have heard that it's a good idea to use it. No problems on the tushy, other than a little numbness and soreness on the sit bones. (We're taking the bikes to get fitted tonight, ladies!!) But I just heard it was advisable to use chamois cream to prevent saddle sores on long long rides (i.e. the MS150). But, again, I'm just worried about using it, because I want no irritation. Does it spread as you sweat? (Sorry if I'm being gross, I don't know who else to ask. I can't imagine asking some bike guy at an LBS. ) Is it like a lotion that you rub in, or does it stay outside the skin as an extra layer?