I bought all 5 seasons, one at a time and in order, from amazon.ca, which did offer them much cheaper than in stores. The price is lower now than what I paid!!! Others have noted the significant difference in price if ordering through Amazon.com instead.
The shows are great, the remastered picture and sound is good, and there's many special features. Season 1 even came with a book about the entire seasons, including reviews of every episode --- I enjoyed that.
The best seasons are 1, 2 and 3. Seasons 4 & 5 each have some great episodes, but fewer than the first 3 seasons. Season 4 had one-hour episodes (while the other seasons had half hour episodes) and it's the weakest season by far.
If you loved this show but aren't sure about spending the money, get the first 3 seasons only. If you want it all, get this definitive collection with all 5 seasons together and save a lot of money. Ultimately, I wish I had went this route, but I was being cautious buying one season at a time. The US price makes it a no-brainer, as far as I'm concerned.