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About 95% of them are cr*p and are designed to:
a) harvest submission emails for spam
b) have user submitted content for adsense, etc.
c) earn some extra money for the "upgraded" listings
d) all of the above and then some.
If you do some research you will find most directories either have no internal PR (intentionally or unintentionally) or their links run through redirects designed not to pass PR/link juice.
Will they help your ranking? Maybe a smidge, but for the most part the vast majority of them are useless.
There are a handful of directories I've found (say 30-40) that seem to be worth much in terms of link value(don't ask which...lots of research time involved).
IMHO, it really shouldn't do you any harm to submit via the submission service, but
You will get better results doing it yourself
You will probably not get much traffic from the directories themselves,
If your site is an affiliate site, the majority of established directories will probably reject your site immediately,
You will probably get a few acceptances, and if the directory is nurtured by the owner, then your link from that directory might become worthwhile within six months
However, if your site is definitely an adwords/affiliate site, perhaps you should consider using paid advertising options straight away