I've bought tickets off of Stub Hub in the past. Didn't have any issues, but the last time I bought from Stub Hub was in '07. The only thing I didn't like was we got our tickets shortly before our trip, even though we had bought them long before our trip.
I used to buy only from Stub Hub. Everything worked great and seen a lot of people at the games with the print tickets. I don't use them anymore because they have hidden fees and their site it really bad to use away from home. Tick Pick is also good. I've never had problems with them and they don't have hidden fees. The price is what you pay.
I actually buy most of my tickets on StubHub. I only use it for electronic tickets though, so that I know I'll get them right away. I had an issue one time where the seats were not right, but the StubHub customer service exchanged them for equitable tickets without much issue. It's true that the fees are added on after you select your tickets, but I just keep that in mind when looking at prices.
I'm guessing you are talking about StubHub and not some generic knockoff company. If you are talking StubHub, I've posted many tickets for sale there and bought many tickets at ballparks around the country. Typically it's cheaper than buying direct from MLB.com
Also, many people will add the ticket number so you will be able to download the tickets to your printer immediately. In this way you will know you have the tickets.
Unless you have a lot of time before the game, I would definitely suggest using this.
I will strongly vouch for the legitimacy of the company and their willingness to protect you from fraud. I once had a problem and the CEO helped me to get it resolved.
I've used StubHub quite a bit for many years, not only for baseball, but basketball & football as well. Their fees are quite steep, they're not necessarily a "good deal", but their tickets are legit and their customer service is top notch.