I have been taking daily celery seed capsules now since March. I take a minimum 1000mg dose each morning and sometimes a second dose in the evening. I try to get vegetarian capsules (on Amazon) as some manufacturers use bovine gelatin in the ingredients, but that's up to personal preference.
The obvious advantage of caps over raw seeds is you avoid the bitter taste of the seeds, but they are more expensive. I tried the raw seeds for a while but found the process of grinding them and brewing in hot water too time consuming and I really hated the taste. So it's caps for me all the way.
As for the effect, CS for me improves both strength of erection, quantity of precum and general libido. Since starting on the caps, I've tested the placebo effect by taking short, weekly breaks from it and when I restart the effects are noticeable. In short, I wake every morning with a really powerful boner and when I edge the precum flows.
I've tried quite a few other supplements, with varying degrees of success, but I am confident that CS caps genuinely help.