You can access a report as normal and view it in the browser, eg the sample report and on there you will now see a direct link to the XML version of the report.
If you know the name of the URL you want to report against then you simple access it via http://jserrlog.appspot.com/report.xml?sn=http://blog.offbeatmammal.com where the parameter after the sn is the URL you want to query.
Both the report and the XML show up to the last 500 results for the URL. I plan to add limits to the XML feed, and pagination to the HTML report in a future release (let me know in the comments what's more important, and any other requests). I would like to implement a full OData feed for the data but haven't found a good Python / App Engine sample out there yet…
One great thing about having the data available as an XML source is that you can add it as a Data Source in Excel and from there filter and sort to your hearts content