I've seen plenty of negative comments regarding performance and general bugginess. However, to be fair, most of these look like they were within the v5.3 timeframe. Have they fixed all of those issues in v6.0? Is it an excellent product?

Some examples of the complaints:

Maybe it’s just a case of user error, but this guy says, “some pages take as much as 20s to render…” Source

Here, in the comments, one fellow remaks, “Sitecore backend is incredible slow. Sitecore developement is really pain, it takes from 2 minutes to start sitecore and many many seconds to do small backend operations. They claim to have a quick client, but that is a BIG LIE. All developers in my company really hate sitecore for being so slow.” Source

Another search yielded, “Sitecore’s users listed three issues as number one: licensing, the server as a resource hog, and the site’s slow responsiveness.” Source