Over the last 15 years, we have bought scores of BPSs of every size and type for ourselves and our clients, nearly all of them from American Power Conversion (APC) (http://www.apc.com). We have had few problems with APC units, something we can't say for units we've used from some competitors.
For current recommendations by brand and model, visit: http://www.hardwareguys.com/picks/bps.html.