Hi,
If you already have a job, a major hurdle is already cleared and you can deal with the rest.
Is it necessary to speak French in Montreal? No. Is it dysfunctional not to? Yes, I think so. If you want to learn French, there are many opportunities to do so. You can take university classes, go to a language school, place an ad and do a language exchange with a francophone who wants to learn English, and/or go through the francisation program. You can find out more about this program here: http://www.immigration-quebec.gouv.qc.ca/en/french-language/learning-online/admission.html.
There is alot of data suggesting that immigrants who go through francisation integrate better and are more upwardly mobile. I know first hand that these courses go a long way in making you functional in a francophone society. The program is completely free.
In terms of quality of life, taxes are high I won't lie. But there are alot of tax breaks for people who are in special situatations for example if you are studying, if your spouse does not work, if you have children, if you are renovating a home etc. If you are smart about it, you can also reduce your tax liability by investing in RRSPs, like in the rest of Canada. It is very inexpensive to buy a condo in Montreal. Condos in nice areas start at around $160,000, which puts ownership well within reach.
I would have to disagree with Charlotte on Montreal being a living museum. This is a very young city with lots of opportunities to socialize and go out. Montreal is known as North America's premier party city and I would say it lives up to this reputation. People here put the emphasis on being happy first. Everything else comes after, so this make it a very pleasant place to live. If you already have a job, I would say come on up. You will not regret it.