concept of the rain barrel is great, never been a fan though for a couple reasons. Both can be overcome if you are that interested.

1. They really don't hold much water. Seems like a lot, but 55 gals doesn't go very far.
2. See above, if you live somewhere that gets heavy downpours, it will fill in mins, then start overflowing

I've seen people put 2 or 3 in series to catch the overflow, but the last one needs to be set up to properly run off the overflow (they go up against your house, so if you don't plan for it, you will likely be overflowing against your foundation). The overflow built into most is the size of a garden hose, will be overwhelmed during a downpour, so make sure you have it set up so the lid or funnels the overflow where you want it and can control it.

Other thing to account for in setting up is where the water is going to be used. Make sure the barrel(s) are above that point so gravity will get it there, or you will need a pump.