I agree that a DIY 50 foot coil with messy bends is better than a pretty ready made 25 foot. The cooling capacity is significant. Its simple surface area for heat transfer.
I have used both 25 and 50 foot copper tube for hundreds of gallons of brewing. I am now a zealot for flat plate chillers. Not only is it hard to get a good lid seal while you are cooling, using the immersion chillers, you will never get a crisp aromatic hop flavor in your face if it takes you 20+ minutes to cool. Plus the fact that fitting a lid on while cooling, and the surface area contact in your sweet wort...the wasted water...
I know some of you who swear by your immersion chillers will disagree, but having made the transition to flat plate, as far as Im concerned, anyone who has invested enough in a 5 gallon kettle with valves and a propane burner, can spare the extra $30 to do it right the first time with a Shirron Flat plate Chiller - especailly if right now they have niether as this post implies.
They are as cheap as ~$95 online (inc shipping!). A bazooka screen in your boil kettle lets you pour directly into your fermenter as fast as gravity will help you (about 6ish minutes depending on your tap water temp).
Besides the time saved, the primary advantage is the quality of your beer. From the time of your last hop addition, your lid sits tight and wort will not see open air (and all the nasties wanting to compete with your yeast) until you rack into your secondary. By then the alcohol will kill them.
Finally, in my experience, if you ever want to consider brewing more than 5 gallons (maybe two 5 gallon batches side by side) the copper immersion is simply inadequate. If you have never used a flat plate chiller, as far as Im concerned having used both extensively, you are just defending the copper without basis for comparison and should consider dumping it on ebay for some newbie to get started with, and use the money to offset getting yourself set up with the supreme luxury of simply opening your valve and dumping directly into your fermenter.