Since I never do ANYTHING without a cover (well, ok, I kiss without a condom ha ha), I feel qualified to answer this.

I think unprotected sex is pretty dumb.  Granted, the recevier of oral doesn't have to worry too much about AIDS, but there is still Herpes, Gonorrhea, Syphilis, Chlamydia, etc.  Now, If you don't have a significant other, then you don't need to worry too much about any of them except Herpes.  If you get tested regularly, you can be cured.

For those like me who have a significant other, condoms must be worn for everything.  Bringing home a bug for the Mrs. is just not an option.  Some providers understand this and are very cool about it.  Others get really weird about it for some reason.  It's almost like they want to share their favorite disease with you.  I understand many covers don't taste good, but geez, you don't have to get rude about it.  I buy flavored ones for the girls who don't mind a cover.

Regular vs. Ultrathin?  Believe it or not, I have found that it is more a matter of the lady's technique.  A lady who knows what she is doing can get me off fast with a thick condom.  And a lady who isn't very good has a hard time even with a thin condom.

One of my better experiences was with a girl at a local AMP who put K-Y on me before applying the cover.  It help things slide around a little more and definitely enhanced the sensation.  I recommend you experiment with this (don't use too much, just a thin coating).

I have not had a provided complain when I've brought my own covers.  It was especially appreciated when I brought my own flavored condoms.

Regarding full service, I know it doesn't feel quite as good, but it definitely comes down to the lady.  If there is a lot of rubbing and snuggling and kissing, then the experience is greatly enhanced.  My current atf at my favorite amp kisses me a lot (tongue and all) and this revs me up like crazy.  I have no problem with any sensation in sessions with her