Great improvement for quickly browsing Reddit. I'm an occasional lurker, so it's perfect for me. I imagine regulars would find it useful too.
This is a great example of a simple small script that has a huge impact on a person's interaction with a website. It creates a huge efficiency when navigating - saves tons of time.
Only enhancement I would suggest, would be to have a parameter which would show more than one (maybe 2-5) top comments.
See this other similar script, which doesn't well work anymore, http://userscripts.org/scripts/show/88045