It's as good as the best air coolers and it's absolutely silent. What it's not good at is cooling graphics cards and chipsets at the same time. Also have a look at the Preytek Serenity. It's half the price and just as good. It also comes in a 'bulk' format so you can buy 2 or more radiators rather than one, have multiple loops and pick your own pumps, blocks and tubing sizes.
Also - my own personal favourite is the Cape Cora 10 Passiv - that'll cool anything you want to throw at it. Again, about the same price as the Zalman once it's configured with multiple blocks and pumps.