This app is amazing in concept, less so in execution. It has serious stability issues on my computer (10.4.10 MacBook Pro). I'm running the 10-day evaluation period now, and in that time it's crashed 4 or 5 times. A rather poor show from an app that people depend on to notify them of server outages anywhere, anytime.
The good news is that this app does what it's intended for, and does it well. I have it checking all my servers, plus my servers' IPs in case of nameserver issues, and (when it's running) it gets the job done quite nicely.
Its processor usage runs around 5% when updating, 0.0% the rest of the time. The memory usage is on the high side, though - 27 MB. By contrast, QuickSilver uses 13 MB (with menu enabled, just like Server Siren) and Growl uses a whopping 7 MB. So I'd definitely like to see this app optimized.
Finally, for the wishlist. I would very much like to see Growl support added. The notifications Server Siren uses are very classy, but Growl is much more flexible and I like to keep everything consistent. An option in the preferences to do this would be nice. What's more, I'd like to see an option to ping a server with near-100% uptime, like Apple.com, before alerting - I don't want to hear that all my servers are down, only to find out it was just my computer's internet that was out.
Server Siren is a nice app and I would certainly buy it if the stability issues were fixed, even though there are many open-source apps (even smaller projects) with a more extensive feature set.
Finally, I'll close with a link to an app with a similar purpose, although somewhat complimentary to Server Siren. I'm still working out how to set it up, but it should be interesting to get it in action.