Premium is indeed interesting for people who want to buy more than one of the upcoming DLCs, including the already-released Back to Karkand (BTW, I wonder whether Aftermath will be a SP expansion or if there will be an additional MP pack with extended hand-to-hand combat techniques or whatever; End Game reportedly takes place in the jungle but does feature tandem motorcycles for fast action).
It's a buy-it-once-and-receive-the-entire-stuff-progressively offer, with unique privileges such as two-week headstarts (the one for CQ is winding down, I wonder whether there'll be a player surge in the next couple of days) and exclusive stuff such as double XP events (there is another one this weekend, btw - looks like last week's was really popular). Aside from that, you just get new black-and-gold dog tags (unlocked through five new assignments), weapon/player camouflage patterns, developer videos (sneak peeks for the upcoming DLCs, gameplay tips, etc), strategy guides and "bonus content drops" (not sure what this is yet). You are also granted a one-time reset of your Win/Loss or Kill/Deaths counters to...make your stats shine better (thought it's almost cheating to me).
You additionally unlock the (fictional) ACB-90 knife, but it's just a cosmetic replacement for the default Ka-bar bowie knife. AFAIK, it isn't any deadlier than the weapon it's supposed to replace, it's just showier due to the orange handle.
And...you are also granted fast-tracking for server queues. Even if it does sound unfair for non-Premium members, the nickel you doled out secures you a place at the top of the player entry priority list. Just think of it as cutting queues when trying to get into a nightclub - yeah, sounds and looks unfair, but don't complain about it.
http://www.battlefield.com/battlefield3/1/premium
So, if you want to acquire all five expansion packs (B2K, CQ, Armored Kill, Aftermath and End Game) at, what? $49.99 instead of $74.95 (assuming each pack costs an average of $14.99 and you want to buy all five of them separately), it's a bargain. Heck, even if you already have B2K, four packs amounts to $59.96 (VAT included, I think) total. So, you can save at least ten dollars...and more if you have promo codes/coupons/whatever.
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But in the end it's all up to you - if you only want to download one or two of the upcoming packs (CQ included) because you don't like the sound of Aftermath, eschew Armored Kill's tank warfare or dread CQ's don't-stop-running-and-gunning-or-you'll-die mayhem, it might be smarter to buy the packs you're interested in
separately instead of ordering the entire Premium pack.