I was on Citalopram for about 9 months. For the first couple of weeks I had a few side effects (nausea, constipation, diarrhoea, wooly-headedness etc) but then settled down well. The only long-term side-effect I had was a feeling of having an electric shock in my head every time I turned it. That's apparently an unusual side-effect with Citalopram though.
20mg as a starting dose will probably mean they will increase the dose later; when mine was increased I had a couple of days where I felt a bit nauseous but nothing more than that.
I stopped taking them pretty easily, I was meant to be changing to a different ad because Citalopram didn't really help me much, and I just reduced the dose slowly and came off them completely over a couple of weeks. No problems with withdrawal, just a bit of nausea for a couple of days when I first dropped the dose.
My doctor prefers to prescribe Citalopram as it's well tolerated, less side-effects and withdrawal is easier than with many of the other SSRIs, and certainly once I got over the first couple of weeks I had no real problems with them (apart from the fact they didn't work for me - but that doesn't mean they won't be right for you).
Hope you get on OK with them.