Foxfire, I am not PlainJaneDoe, but I will take a stab at your question.
Meth, in regard to the brain, affects the receptors in the brain causing them to release a massive surge of dopamine which causes a rush of pleasure sensation and a feeling of euphoria. The pleasure lasts for several minutes, and the euphoria lasts for several hours (6-12 hours depending on how much was ingested and how tolerant you are to the drug). Funny thing is, after meth destroys these receptors over time, it leaves the person unable to feel any pleasure. Ironic, no? It will change the brain chemistry of an addict over time.
It also stimulates a release of adrenaline. It increase anxiety, inability to sleep (sometimes for days), or increased attention. This is what is refered to as "tweaking". It makes the user hyperactive and obsessive. It can cause hallucinations and paranoia in heavy users. Some users get very agressive. It also causes an increase in confidence and sexual prowess and an idea that they are extremely desireable. Once they come down it is a low depression and they need to use again to achieve that pleasurable feeling. That is why it is so addictive.
The chemicals that meth is made out of are very poisonous and something not natural to the human body in any way. These toxins have to find a way out of the body (the body doesn't know what to do with them) and it is secreted in other than the normal ways such as the skin. That's why some tweakers have the sores that are common with meth abuse.
As for the sexual predator crimes, that ties in with the feelings of sexual prowess. I have been in plenty of houses where meth users reside and it is mind blowing. There is almost always a vast collection of *advertiser censored* (purchased and homemade) in the house and more often than not it is playing on all the televisions in the entire house 24/7. And this isn't your typical Playboy channel stuff either. This is the most disgusting stuff you can lay eyes on. There are sex toys and lubricants scattered everywhere in plain (and handy) view. It's all about the pleasure for these people. And sadly, many of these homes have children residing there. That's truly the worst part.
I apologize to PlainJaneDoe. I don't mean to step on your toes in any way, just thought I would answer this post. I have going on 10 years experience working for a narcotic law enforcement agency, so that is my knowledge base.
For a grant im writing, ive been doing a lot of reading about how psychostimulants have complex effects on serotonin in large part because of how they affect different populations of receptors at different doses. Hence, the range of effects seen on behaviors associated with anxiety, defense, aggression, and response to natural reward.