People are using Mt. Gox, because... people are using Mt. Gox. The market got big because of few factors and now if you want to trade on a highly dynamic market, in real time, this is the place for that. But it's not the best for all. For example, here in Poland there is a far better exchange - Bitomat.pl
(trading Polish zloty only). It's better, because it's totally free, and your wire money transfer is acknowledged within hours (sometimes even minutes) from sending. These transfers are also free in Poland.
On the other hand, Bitmarket.eu
has a different, less dynamic model. You can place offers just like on MtGox, but to exchange actual cash, you have to contact with the other party. This has several advantages, like more anonymity (only the other party knows your private details, you don't expose them to the site), faster first reaction (you don't have to wait for your funds to come up on the site to trade) and more control. It has some disadvantages as well - most importantly - you lose the "real-time" bit, because every trade needs to be handled manually (in terms of exchanging cash). There is also a risk (albeit a small one because of verification system, and soon - rating system) of a fraudulent seller/buyer, but in that case you can contact site support and we will help you.
If you have other questions, feel free to ask.