Greylisting will effectively stop lots of spam before it even hits your content filter.
It is a really useful addiction because it will greatly reduce your scanning workload, will reduce false negatives (some of the spam that wouldn't get caught by your content filter will be blocked beforehand by greylisting), and it cannot, by definition, introduce any false positive (legitimate mail being blocked).
Mails you loose are due to not conforming smtp senders - yes there are some "bigs" still not playing nice, a short whitelist will take care of them until they fix their systems. In the end, having lots of sites with greylisting on the internet will have the nice side effect of forcing more people to use correctly configured mail servers.
With a good greylist setup (good implementation + good configuration/operations) very few mails will get delayed, and most of the time the delay will be in the order of a few minutes. Also, a good greylisting setup is mostly a "deploy and forget" system, reducing spam flow, systems load while not increasing your (sysadmin) load.
Before actually turning on greylisting on existing domains I strongly suggest to deploy it in "learning mode", where it'll watch the mail flow without delaying anything. That will give it time to learn triplets and autowhitelist good smtp senders.
Having lots of mails getting blocked before the content scanner will have a number of good side effects. I particularly like these:
- other than the short and infrequently-changing manual whitelists, a greylisting system does not need any shared knowledge between servers, simplifying the deploy of multiple MX'es
in geografically distributed locations/datacenters
- reducing the scanning load means you can use less hardware for content scanning
- less servers for content scanning means you can more easily centralize them, manage them, debug them (better signal/noise ratio in the logs ;)
- less load on your systems to reject 'obvious' spam and more load on a spammer system to retry delivery both mean a better receiver load / spammer load ratio, that makes sending spam more 'expensive', and this is a good thing in the long term
All in all, greylisting boils down to:
- forcing senders to conform to standards, this will make it easier for the whole email system to work correctly and be more easily manageable (--> more easily tracking down spammers as a side effect)
- increasing (a little) the cost of email sending, having a little impact on legitimate senders and a bigger one on spammers (--> increasing spam sending costs is always good)
EDIT: while there is an (small, but that's IMHO) impact of legitimate mail delivery times, it could be reduced by using other means to bypass greylisting, like tarpitting and SPF. The former is interesting but I'd do some real world tests before judging its effectiveness / drawbacks, the latter isn't always available.