Don't forget you get the OST.
Indeed. With most recent games, the AC series included, selling their soundtracks separately, the soundtrack becomes a fairly hefty bonus.
the soundtrack is something you want and don't have, then compared to buying game & MP3s individually through other services it isn't that much more expensive. Still more than you'd pay by waiting for a Steam sale + getting the soundtrack off Amazon, but only about $5 more.
I'm currently in wait-for-a-sale mode, myself, as I already bit on this series in DRMed form and the first is certainly the worst of the series (though hardly bad). But I'm glad to see it on GoG, and if Ubisoft shows they're serious about bringing the whole AC series to GoG, I'll definitely buy them all on here at some point or another.