I looked into oral irrigators after some particularly painful and expensive dental treatment.
I tried the QuickBreeze Dental Irrigator in the USA, which I really liked. It was a good price, too. However, it connects to the tap, and wasn't compatible with my taps in the UK.
I first thought about the portable options, but the reviews weren't very good regarding battery life / capacity. So I plumped for the Kitty Hydro Floss Water Jet.
Although more expensive than most other oral irrigators, and perhaps not as powerful as the QuickBreeze, it is still very effective, and quick and easy to use. I use it twice a day, with warm water, after brushing and flossing, and it leaves my mouth feeling clean and fresh. One fill is plenty for one use. It is also sturdy, and comes with a handy bracket so that it can be wall mounted. Highly recommended!
My dentist noticed a general improvement in my teeth during a check-up in June 2009. Above all, she said some gaps that had been appearing between my teeth had become smaller. She asked me what I was using. I told her that I had been using the Kitty Hydro Floss Water Jet for six months, in addition to my normal routine of an oscillating electric toothbrush and flossing. She said she doesn't normally recommend oral irrigation systems, and liked to think that the improvement was just down to the toothbrush and flossing. But she advised me to keep doing whatever I'm doing, as it's definitely working!