http://www.duelingmixes.com/
 
I have been looking at this on and off for a while as an interesting way to look at improving my mixing skills and  the recent what have we become thread reminded me that it was on my radar.
 
Has anyone tried this or are using it?

I prefer to purchase my courses outright rather then subscribe...

Udemy is always having sales and they have really good tutorials...
as well as Ask and MacPro...

Not too keen on a "subscription based" tutorial service...but I guess you
can cancel at any time...so do it for a month...and if you don't like it dump it...

I've been a Dueling Mixes member for about a year. I believe it has helped me quite a lot. You get the raw tracks to mix, a tutorial from each guy (Graham and Joe) with every new mix as well as their mixes to reference against yours if you like and a web based conference call/webinar where you can ask questions on this month's song or really anything you might want.
 
The songs vary, with some being really well recorded and excellent songs, but lean toward the mellow side in general, from country/folk-ish to pop to classic and modern rock (no real metal songs since I've been a member though).
 
So each month, you get two perspectives on how to mix the song as well as a fairly active forum to post your mix and get feedback from other members.
 
To me, it's been well worth the money. I get extra practice mixing genres I wouldn't ordinarily do, as well as compare my mix to Joe's and  Graham's.

I'm probably going to start soon.  I listen to both their podcasts and youtube channels and have learned a TON.

Yeah the thing I like and are making me think seriously about it is that you get two people giving there insight not just one. You get to keep the raw tracks to do your own mixes on the same material as they are using. Also you have access and can ask direct questions.
 
I may just join for a couple of months to try it out and see how it goes.