I do agree that there are some questionable sites with PR4 and above but a little check will more often show that there used to be a history with such sites. In most of my surfing around, genuine sites with PR4 and above (especially PR5 upwards) have good content and have much effort put into it. You can clearly see the difference.
Any PR is good as long as you are getting traffic. I think PR4 or PR5 are what is really good for any site. However, the real thing that matters is the traffic.
I don't give much importance to PR although I get a little bit excited when there is an update... I bother more about traffic - targeted (organic!)
I reached till PR5 for one of my blog. More than PR5 will be exciting, my other blogs are also PR4 and PR3.
The higher the better, but as someone said earlier, it is just a method recognition. If you do get a good PR it might give you some boost in Google's rankings. But even with a low PR, you could still rank well.
Actually, when a site has high PR, chances are it will rank high on search results. When my PR4 blog was slapped with a PR0, the referral traffic from search engines dropped tremendously. Later, after the blog was reinstated with a PR4, the incoming traffic went back to normal and is in fact getting stronger
as a newbie if you get a PR of 3 or 4 this will be good for your site. However your serps rating is much more critical and this is based on building backlinks to your site, which then generates traffic