I'm using the product, and have done so for a couple of years now. Yes, it was a pain to configure but what accounting software isn't? At least I don't have to worry about installation, patching, backups, and general maintenance.
It has most definitely saved me money, with my accountant charging me 1/10 of what they had been charging for completing my GST return, prior to using this software. The hidden cost however, is the time it takes me to personally administer the accounts.
Wave is something I haven't had a chance to look at yet, but will do so over the coming weeks. My real concern about the product is whether this free model can generate the revenue streams required to invest back into product development. The other issue is whether they can resist using data analysis to target adverts to users. Got a spare $5K sitting in your account? Maybe it's time to serve up a advert for a new HP Server costing $4,999? Perhaps a "freemium" based service will ultimately how the product evolves.