I don't know if I'd say it's a great deal. He's a top 5 talent at tackle, and he's getting paid like one. In terms of salary on a per-year basis, he's the second highest paid tackle ever.
He deserves it, but that's the going rate for a guy at his level.
Comparatively, it is a good deal when you consider Cordy Glenn got an almost identical contract. Glenn is a solid player, but a tier below Armstead.