PROS:
-The TA magazine is much better in person. Sometimes pages don't even get scanned. When they do, the texture of the page means they often scan badly.
-OMG THE STUFF. TA members get stuff every so often throughout the year - random novelties. Since becoming a member I've gotten an A logo headphone clip, a keyring, a flashlight that shines the TeamAyu logo in a circle, magnets, new year's & other random cards, a charm, the 15th anniversary puzzle... it's good times. Ordering from mu-mo gets you fun TA logo stuff if you like, too, or the mu-mo first press item is an option too.
-Access to the TA section of her official site. It doesn't have as much as it used to, but now you don't have to wait for other fans to upload screensavers, and if you learn Japanese you can read ayu's blog/diary entries yourself instead of waiting for translations.
-Access to the FC website at fc.avex.jp/ayu -> this one has older news stories, copies of the emails you get, and things like...
-Giveaways. You can win things from time to time, as with the 15th Anniversary Live quiz that happened recently. If you want a shot, get your magazine out of your tenso box ASAP though, because they're almost always announced in TA magazine - I frequently miss giveaways because of this x_x
Cons:
-The website is split between fc.avex.jp and teamayu.com-slash-ayu's-official-site (which is one site now). It's hard to navigate, especially if your japanese is not so good.
-Fans keep uploading the screensavers, which makes the download part of the website not so worth it.
-If you want the TeamAyu first press merchandise AND the mu-mo first press merchandise for a CD that comes out, you have to order two copies - you used to be able to get both back in 2011 and early 2012.
-TA members get a bunch of stuff that really only works if you're in japan, for example, things you can buy or win at concerts. Not to mention first dibs on concert tickets.
-Even if you're in Japan, first dibs on concert tickets doesn't mean automatically good seats.
-Even though Ayu has acknowledged publicly on Twitter that she has overseas fans who are members, and even though Avex & mu-mo have worked with Tenso to help OS fans, they still offer no services in English.
That's about it
I like the stuff and the blog access, to me that's worth it. I do wish they were more understanding about English-speaking fans though.