Scriptish promises several advantages over Greasemonkey, among them better performance, upgradable user scripts, introduction of functions that are only supported by Google Chrome, new Api features, metadata block headers and regular faster updates.
The Firefox extension supports all userscripts that can be loaded into Greasemonkey, and then some that make use of Google chrome functions that Greasemonkey does not support right now.
Probably the biggest problem for existing Greasemonkey users is that Scriptish cannot import userscripts from Greasemonkey, which means that they need to be installed anew before they become available in the new extension.
But that is a one time process and should not take that long, especially since the developer has posted a guide to manually import Greasemonkey userscripts into Scriptish on the official website.
Scriptish offers better manageability of userscripts. It is for instance possible to edit excludes and includes, and a userscript's options in the Firefox add-on manager. Greasemonkey users need to be on one of the pages supported by the userscript to access the options. The extension in addition reveals more information about userscripts directly in the add-on manager.
Is Scriptish the better Greasemonkey? It certainly offers support for features that Greasemonkey does not offer. More interesting than that may be that it offers better access to configuration options and additional information about individual userscripts.
Does it offer a better performance than Greasemonkey? I could not really tell a difference, but that does not mean there is none as it depends a lot on the computer and the scripts that are used.
The developers of the Scriptish extension have posted a page on the official Wiki that details some of the performance benefits of Scriptish compared to Greasemonkey.
Another benefit of Scriptish is that security improvements have been integrated into the extension. You can read more about them here. Note that some of the information date back to 2011.
Scriptish is an option for Greasemonkey users who experience performance problems when running scripts, users who want to run userscripts that are working in Chrome but not in Firefox and users who prefer extensions that receive regular updates. For the rest it is definitely an extension to keep an eye on.