I have a Waterpik cordless and it has been very good - the industrial design is meh but it does the job. It's nice to have a simple handheld unit. I have some gaps between teeth that can trap food and cause gum disease.

It's used in combination with brushing, of course - water-pik then brush.

It's a WF-02 ... it's the kind with 3 AA cells that I have rechargeables in (this is better for my bathroom setup).