Very new to this, only bought the Core Set and the first couple of Adventure packs last week. I've played the first Quest in the Core Set and it's been great fun, I think I've played it around 14-15 times using all the various decks either solo (though really didn't enjoy Tactics) and then starting to mix the decks together and trying out different combos. Moved onto Journey Down the Anduin and it's certainly a lot harder (yay for drawing a second Hill Troll on my first turn...) but I like that I'm having to think more. I've been playing solo, though intorudcing a friend to it (I like solo games, being rather anti-social at times!)
I've since bought what I've been able to (I've got all of Mirkwood bar Emyn Muils, though I gather that's no biggie). Managed to snag Khazad Dum and a couple of Adventure Packs, but I'm hitting the problem I'm having with Black Riders; I can't find some of the packs anywhere. Granted, this is specifically TWitW (which has the fantastic Lore Aragaorn thatI'm struggling to find; Lore has been my favourite Sphere so far) so I've settled for most of Against the Shadow cycle.
I'd definitely recommend, looking at the cards, playing the Core Set and all it's myriad ways of playing a good couple or so dozen times to get a feel for it and then slowly working your way through the APs. I'm going to, as has been suggested above, use only those cycles available at time of release when I play each AP quest.
Best game I've played in a very, very long time :-)