Does the original Bitcoin client scale to contain millions of addresses?
I've ran tests before on .5 million addresses generated in one afternoon. It fired walletnotify within the milliseconds, no noticeable delay when I sent an amount to a random address. Only noticeable thing was that the client took 30 minutes to start up, which I guess is fine if you run a larger service. Only thing that may trouble you is slow transaction creation time, but someone on here advised me to just move the funds into a new address monthly.