jQuery... I have mixed feelings about it.
On the one hand, its very good for high level thinking and writing good, readable code. On the other hand, it slows down your javascript code by at least 2x, which is particularly noticeable if you're doing a javascript intensive application. Then again, the future of browsers is speeding up javascript, so this may not be an issue in a couple of years.
My experience with Prototype is that it handles the speed issue much better - however, it is a bloated monster of a framework.
I don't pretend to know what I'm doing with JS, so I'll keep your advice in mind should I have to figure out why something isn't as fast as it should be.
(I wrote something to this effect on Proggit once, and John Resig reply was something like, "Of course. JQuery's not intended for performance-critical code - usually you don't want to be using any framework for that. JQuery's meant to make the other 90% of JavaScript development go faster.)
BTW, I won't go near Prototype (after having used it pretty extensively at a past employer) because it messes with Object.prototype. When libraries do that, you have no way of knowing which libraries are compatible, which means that you'll suddenly get bitten by very odd, hard-to-track-down bugs.
Now if development time is a greater priority over performance then I think the choice is pretty obvious.
Wittens' talk at http://acko.net/blog/jquery-oscms-presentation-slides at the OSCMS was pretty good; basically, JavaScript is not bad, but too much can be overwhelming to people.
P.S. Not always the slowest set of elements to load on a page. A fun little hack: http://www.spaceminnow.com/school - standard disclaimer -