eval() == evil? --- How to use it safely?

[Fett]

I am creating a program that requires some data that must be kept up
to date. What I plan is to put this data up on a web-site then have
the program periodically pull the data off the web-site.

My problem is that when I pull the data (currently stored as a
dictionary on the site) off the site, it is a string, I can use eval()
to make that string into a dictionary, and everything is great.
However, this means that I am using eval() on some string on a web-
site, which seems pretty un-safe.

I read that by using eval(code,{"__builtins__":None},{}) I can prevent
them from using pretty much anything, and my nested dictionary of
strings is still allowable. What I want to know is:

What are the dangers of eval?
- I originally was using exec() but switched to eval() because I
didn't want some hacker to be able to delete/steal files off my
clients computers. I assume this is not an issue with eval(), since
eval wont execute commands.
- What exactly can someone do by modifying my code string in a command
like: thing = eval(code{"__builtins__":None},{}), anything other than
assign their own values to the object thing?