I use a PUR carbon filter. It's identical to a Brita filter, only it is an adapter that runs the purified water directly from the faucet (think Brita does that also). I live in Seattle and the water is fine to use in most all-grain batches without any filtration, though I do notice a significant improvement in taste with filtration. If you are doing extract batches, then I would suggest picking one of these up. They can do a recommended 100gals of water before needing to be changed. That should save you around 100 dollars and all the effort of dragging bottled water home from the grocery.
Now if you are doing all-grains you would have to know your water's chemistry. Which, in the end, you may ultimately have to carry a few bottles of distilled water on the side.