Just a note: make sure you do the math, as not all reels are created equally. Some reels are sold in 200m, 220m, 660ft or 726ft. Here's the conversions:
200m = 656.17ft
220m = 721.78ft
While the differences are seemingly trivial, there are two points here. If you are stringing for your racquet, and your racquet alone, knowing exactly how much string you need can save you money. Instead of cutting off 40ft of string like is in a normal set, you can cut just enough which is often a few feet less. If you do not do this and simply cut 40ft every time, you likely won't save money at all over the price of individual sets. The second point is in the reel itself. Depending on its length, the price/stringjob can be very different. Many reels are priced similarly as a reel, not based on the length. More importantly, many reels are priced by set rather than priced for being a value. Here's examples:
Technifibre X-1 Biphase, 660 foot reel. $250. That yields 16.5 40 foot sets. Price per set: $15.15 bucks.
Technifibre X-1 Biphase 40ft set. $16/set.
If you do the math, that means you end up saving a total of $14. In other words, unless you're completely committed to X-1, you're better off buying sets.
Now, here's a good example of savings:
WeissCANNON Mosquito Bite 660 foot reel. $99.90. Price per 40ft set: $6.05
WeissCANNON Mosquito Bite 40ft set. $9.95.
Now, if you do the math here, you end up saving $64.28. That's a LOT of savings. WC Black5Edge can be had in a reel and when it came out, the savings were about $98 over the initial $12/set price. That's because it was priced at $99.90 like the other WC reels. I bought that reel without even thinking about it. So you see, it really depends on your string choices as to whether or not a reel is a good investment. If you string frequently, you have nothing to lose by getting a reel. Not only that, but it's more convenient than having sets lying around. If you switch strings often or string for many people who like different strings, then having reels is not always beneficial.
As a fun fact: I bought my reel of Signum Pro Tornado from England for $79.15 ex VAT. Why? It's 200m, which I'll round to 660. Sets here in the states cost 14.69. That means that I just saved $163.24, which is less than $14 different than the reel costs to buy in America. In other words, I bought 2.23 reels of the string in the UK for the price of one in the US. Shipping was only 5.95 too. Moral: pays to look around when there's a string you really like ;)