Stayed here three times and I am afraid I must issue a warning: they tried to charge extra every single time. Apart from that, the pros and cons are,

Pros:
* convenient location in Itaim Bibi business district
* nowadays free WiFi (extra speed cost more but was not necessary)
* good breakfast
* straightforward rooms but with good bed and enough light (for work)

Cons:
* cleanliness could be improved
* no heater (in winter it can get quite cold at night in SP)
* three stays and three attempts to rip me off:

1) first time I was there with some colleagues for 12 days. We had to go to Rio for a couple of days and asked for a rebate for the days we would not be there. The rate turned out to be more expensive for 9 days (because we had pre-booked a 10-day stay rate). Not very nice but understandable, so we decided to keep the rooms for the full 12 days and go to Rio. After we returned a sweater was missing from one of our rooms and they denied that there ever was one. It turned out they re-sold the rooms we already paid for to other guests. After a really nasty discussion we got the sweater back but no rebate nor apologies. So they got paid double for some days in 'our' rooms. I do not now if this is a criminal offence in Brazil, but I find it extremely immoral.
2) Second time they 'just' tried to bill some extra cost. There was a 'mistake' (one day too many, two days tax too many) on the bill as well as mini-bar cost I did not use any of. After some debate and two bills I rejected, the third one was finally correct.
3) Last time I prepaid for three nights and took a fourth night against the higher full flexible rate. To my astonishment, my credit card was billed for a higher amount than the prepaid rate, without advance warning or my consent... Accor simple responded that I had to contact the hotel for this matter (although it was booked on the international Accor site with the alleged pre-paid tariff). Of course, again some 'mistakes' while checking out. This time they tried to charge the room tax for four nights (so also the tax I had already pre-paid), added room service and mini-bar usage again. So another debate, and the third printed receipt seemd right, although still higher than the rate as advertised when I reserved.

Accor: investigate this matter, there is something structurally wrong because it happened three times at this specific Mercure, and nowhere else in Brazil so far.

Avoid or at least be very aware!