I use one of them and I try and only use one extra besides what Google offers. If I use more, I get all kinds of weird issues with one app trying to install over another installed (and updated app) because its service was not the one that installed the original application. I run into this a lot with Amazon apps I have installed.
Google’s Android Market isn’t the only place you can get Android apps. Whether you’re looking for free paid apps, social recommendations or an app store to replace a missing Android Market, you have a lot of choice.
Each app store is completely legit — the ones that offer free apps pay the developer for the privilege. You may even have an alternative app store on your device already if you’re an Opera user.