I'd say - GET THEM!!!
The Tanteidan magazines are by far the most high-quality origami related publication in the world. The only down side is that they are in Japanese, but the diagrams are so crystal clear that it's not that big a downside. They include lots of articles which I would love to be able to read, but the diagrams themselves are well worth the cost.
There are usually two sets of diagrams in each magazine. The first two diagrams are simple-intermediate modular/geometric models, and the last two are (mega)complex models, usually one with diagrams and one with a crease-pattern (that includes hints in Japanese

).
About ordering, there is a very simple method to order, but I will only get to it after I get to tell my shocking true story. When I first wanted to order them, I went to the bank to get an "international postal money order". The folks at the bank had no idea what the hell this was, so they gave me an international check, or something like it. Needless to say, after several weeks of anticipation, it came back. That's probably when I asked Gilad Aharoni how he orders them, and he told me of the easy way to do it.
Oh, yah, I guess you want to know how
Well, if you don't already have a PayPal account set, make one (it's very easy). Just go to
http://www.paypal.com/
Next, email June Sakamoto, of OUSA, who, volunteers to arrange orders from JOAS. You can pay with PayPal (with a very small extra to cover paypal stuff). At least dozens of people order through her and I don't know of anyone who was disappointed. I would just like to repeat that she does all this on her free time and dons't get a cent in return, so a BIG "THANK YOU JUNE" is in order.
For her email, please message me privately.