If you've ever used SharePoint Designer, you'll know it's bad. Really bad.
Is there any good replacement for that? Is there any consideration I must take account of if I want to build my own Visual Studio Add-in connector to connect to it?
Anything you know that I should be aware of?
I just hate the slowness and the way SPD (SharePoint Designer) just keeps reformating all my stuff.
You can get the Sharepoint Explorer add-in for Visual Studio which will allow you to browse around sites:
It depends on what you want to do. As Cruiser mentioned, there are options like SharePoint Explorer for exploring a SharePoint farm or site collection. However there are a lot of things that there is no replacement for such as SP Designer workflows, editing page layouts, etc...