I tend to answer this on the basis of "you should be able to see for yourself". It helps of course to have the right suggestions as to how you look to "see" the right things.
So, let's look first at leakage. Holes are pretty uncommon nowadays in commercial condoms (they have quite sophisticated machines to test them), as long as no-one has been near them with a needle and they haven't been opened before, exposed to excess heat for a long time (such as in a car glovebox - they are cooler in your top pocket), or exposed to any sort of oil (use only lubricants designed and marketed to be compatible with condoms).
Now as Blastoff says, there is a situation when you actually ejaculate, that if you keep on thrusting too long after ejaculation, semen will eventually get "pumped" along the sides, from where it could eventually run back over the outside, and from there be wiped inside the girl. This is especially likely if you lose your erection but keep pumping, and the condom is also at risk of slipping off and being pushed completely inside her, semen and all. Thus you are advised to grab it (the base of the condom) and pull out immediately after you "come".
If you want to (and actually can!) keep pumping afterward, you would be better to use the "female condom" or "Femidom" which is a really big one which goes - and stays - inside the girl, so it can't slip just because you go limp. It has other advantages too!
As to "pre-***", I have explained that in detail some time ago, on the [url="http://www.healthboards.com/ubb/Forum116/HTML/000830.html"]Sexual health - Women"[/url] board. It is not semen and does not contain even remotely enough sperm to cause pregnancy unless of course, you've previously released some semen (that is, you have "come" either fully or "just a little bit" beforehand, and haven't peed to flush it all out yet).
So whether or not you've noticed "pre-***", rubbing your penis on her bits is not a risk of pregnancy unless you feel it start to come. It is however, the most effective way to exchange any STD that either of you might happen to have, so do bear that in mind. The vast majority of STDs are less likely to be present in the semen proper, than in "pre-***".
Effectiveness of condoms? Figures depend on how much allowance is made of breakage, falling off, and outright lying (i.e., just weren't actually used!). Outside of those three things, at least 97% effective over a year .
[This message has been edited by paulb (edited 01-04-2003).]