that website is only good if you want to learn how base58 encoding works and only if you want to play around with some keys that you are not using. but there is no source code for it, so it may contain bugs or ever worse it may be recording every private key that you enter in it online so it is not at all secure.
for a better tool you should always suggest bitaddress.org it is open source, you can see the source and download on github and also it is tested so the chances of bugs are nearly zero. it accepts all formats: hex, base58, base64 and BIP38 encrypted