Martin has 2 great points. I started with a Therminator when they first came out, but it can't cool the wort any colder than your tap water. So in the Summers, when I brew lagers, I got a Duda 20-plate chiller that I put in-line. The first chiller gets tap water and the second gets ice water pumped through it with a little sump pump. Works like a charm.
And I highly recommend placing a thermometer on the wort output and use valves on both the water and the wort so you dial in the wort temperature.
So if you live in the Great White North, I would recommend one of the 20-plate chillers. If you brew sometimes when your tap water is warmer than you want the wort, get a second 20-plate chiller. It's still a lot cheaper than a Therminator or a Chillzilla.