In my opinion, yes it is worth it.
You can start from 150M and go up anytime you want. You can also go down anytime you want.
You will have a history graphic available in the panel to see that. It shows the behavior based on time. Transfer isn't included in the graphic as it is not one of the resource you can change in PS.
To me it was like night and day. My shared server was always behaving very well but it was sometimes little bit slow. Sometimes I was able to get a really good performance (when others weren't doing anything). PS makes all of this very stable, you get what you wanted. You will decide how much memory and CPU you want to use and it is not affected by others.
If I would have been on a server where there are some problematic high resource sites then I guess I would have seen even more dramatic change.
You will receive a secret code to access the place after your site has been transferred to the PS. It is a really cool secret club. Just kidding, we hand out here and also spend the saved time (not monitoring our site so closely) with other things in life.
The whole existing account gets transferred to the PS when you make the change.