As an EU citizen, Macau will give me a 90 day visa upon arrival.
Can I visit Hong Kong with that every other weekend or will leaving Macau void my visa?
Yes, you can visit Hong Kong and re-enter. To be exact, as an EU citizen you do not need a visa for Macau, you will be granted a 90-day entry permit authorization to stay on arrival. Each individual authorization will end when you leave Macau, but you'll be granted a new one when you return.
Beware that, if you keep doing this, you may eventually be refused entry for
(1) Attempting to evade the regulation on stay and residence and having frequent entries to the Macao Special Administrative Region within short intervals without appropriate justification;
So you'd be advised to get a work/study/extended stay permit from Macau if you have reason to stay there longer, and I presume you do, because even a hardcore gambler would get a wee bit bored after 90 days!
According to the official website, entry permits (visa-on-arrival) are only valid for 30 days and you don't need one as an EU citizen. What you'll get is 90 days of visa-free stay.
If you leave Macau, you will need to go through immigration when you come back and you could then theoretically be denied entry.
The same website specifies that
2 Non-residents will be refused entry to the Macao Special Administrative Region for the following reasons:
(1) Attempting to evade the regulation on stay and residence and having frequent entries to the Macao Special Administrative Region within short intervals without appropriate justification;
In practice, staying in Hong Kong and travelling to Macau, I got a new entry stamp/authorization to stay each time very easily. I expect things to work the same the other way around but I don't know how often you can do that before raising suspicion.
I arrived in Macao by plane on April 26th, as a German citizen I got I visa free stay until 25-07-2015 (90 days).
Yesterday (June 5th) I went on a day trip to Honk Kong, when returning to Macau I got a new Arrival Card, it states again:
Permitted to remain until 25-07-2015
This indicates, that the original "visa" was not voided, and the 90 days did not get reset. I'm still on my original 90 days.
Which begs the question, when is it reset? I'm pretty sure last year when I left Macau and returned by plane I got new 90 days. But because there are no stamps for Hong Kong and Macau in my passport (you just get arrival cards) I can not verify that.
And No, I am not gambling here.