First of all, it is cheaper - about $100 difference at present, if you get the basic Surface tablet (though you can get more expensive options with additional features such as the touch cover).
The screen is larger and it has twice as much memory as the iPad. The touch or type cover options have been popular with many users. Another big plus is the full size USB port.
In many other respects it is quite similar to the iPad. Weight, storage capacity etc are about the same, as are Wifi and Bluetooth connections. In some respects the Surface does lose out to the iPad: The screen resolution is much lower (this is said to be because they couldn't increase it without raising the price too much) and the battery is less powerful.
Overall, users have been fairly positive about the Surface, but don't consider it as good as the iPad (then again, it does cost less so you wouldn't expect it to be).
You can see a full list of comparisons here.