It’s important to see some things before chosing a script or buying it:

1. how large the community is. You will have problems, you will need help. A huge community is always good.
2. official support: be it paid or free, the script needs to have a developmentn team ready to assist you
3. skinning possibilities: I hate scripts that limit my templating skills. I always chose the ones I can skin easier
4. security: how many attacks does a script attract, how to solve the exploits etc.
5. money: we can’t afford some huge priced ones.

I always thing about these 5 things and then it’s easier.

Some are worth buying, some are not, just as you said.