Well, as for performance release post says:
In this release we’ve also been able
to improve the performance of some
commonly-used traversal methods:
.children(), .prev(), and .next(). The
speed-ups that we’re seeing are quite
substantial (potentially many many
times faster, depending upon the
browser).
On other hand replacing jQuery on admin side is rarely good idea, because it is getting merged into concatenated bunch of scripts and it’s a mess to deal with.
Unless you are suffering from considerable JS performance issues in admin, my personal opinion is that it isn’t worth the trouble.
PS WP 3.1 will have newer 1.4.4 jQuery version, don’t know if they will bother to bring it up to 1.5 by final release.
Update
WordPress 3.0.5 was released at the
same time as jQuery 1.5.
Unfortunately, 1.5 has some backwards
incompatible changes that appear to
break a number of areas in the admin.
The timing is awkward and it looks
like it was us. It wasn’t.There’s nothing we can do about this even for WordPress 3.1, which is
freezing at jQuery 1.4.4.( Andrew Nacin )
So issues t31os desribed are definitely caused by jQuery 1.5 and it is definitely not recommended to use in admin area at moment.