The mining should run at about 70% speed compared to a dedicated software miner.
(Problem is compiled c running on JS rather than compiled c running on hardware)
Cryptonight doesn't have asics yet, so it is better than sha256, or scrypt, profit wise.
I believe they use coinhive, which then takes 30% of that profits.
So you really get limited as to how much you can earn, because of the nickeling and diming.
I wasn't really asked here, but my latest site is running a promotion where we payout 200% of what Coinhive pays us. Can PM my site to anyone interested, but don't want to spam where I wasn't asked.
All in all, no. It isn't going to make you rich, or possibly even break even.
But it can be a good way to get your hands on some crypto dust without actually having to physically "pay" for it.
(You just pay with shortened hardware life/electric fees.)