This will be pretty interesting. I got into Arch Enemy when Wages of Sin came out and I saw them with Angela at NEMHF '02. Since that was my first exposure to them, I was more "used to" the Angela stuff than the Johan stuff, but liked both eras.
However, I don't think I've heard a "unique and new" Arch Enemy song in about 10 years. Even though they are amazing players (particularly on the guitar side of things) it feels like they are never willing to leave their niche. Sure, some bands can make a career out of that (looks at Slayer, Amon Amarth, etc) but with Arch Enemy the skill of the band members makes you wonder what else they could do.
Alissa is a better overall vocalist than Angela - by far. She can sing, she can get deeper with her growls, she can growl in key, etc. She's also got a bit of a diva reputation, and might not have the stage presence of Angela. Unless AE uses this moment to perhaps open their sound up a bit, I could see things ending quickly. They aren't the draw they were in that 2001-2005 period where they were one of the biggest bands in the underground, and everyone in this band is capable of side gigs that pay some of the bills.