The SQLCAT team just released a very interesting technical note which compares the relative “scalability goodness” of Reporting Services 2005 to 2008. You can read the article just as well as I can, but here’s the executive summary, and the results are pretty impressive (bold is mine, btw)
Reporting Services 2008 was able to respond to 3–4 times the total number of users and their requests on the same hardware without HTTP 503 Service Is Unavailable errors compared with Reporting Services 2005, regardless of the type of renderer. In stark contrast, Reporting Services 2005 generated excessive HTTP 503 Service Is Unavailable errors as the number of users and their requests increased, regardless of the report renderer.
Our tests clearly show that the new memory management architecture of the report server enables Reporting Services 2008 to scale very well, particularly on the new four-processor, quad-core processors. With our test workload, Reporting Services 2008 consistently outperformed SQL Server 2005 with the PDF and XLS renderers on the four-processor, quad-core hardware platform (16 cores) both in terms of response time and in terms of total throughput.
Furthermore, with these renderers on this hardware platform, Reporting Services dramatically outperformed other hardware platforms regardless of Reporting Services version, responding to 3–5 times the number of requests than when running on either of the other hardware platforms. As a result, we recommend that you scale up to four-processor, quad-core servers for performance and scale out to a two-node deployment for high availability. Thereafter, as demand for more capacity occurs, add more four-processor, quad-core servers.
Finally, with all renderers and with all hardware platforms using our test workload, the performance bottlenecks were the processor on the front-end server and the disk subsystem on the data source with Reporting Services 2008, whereas the Reporting Services front-end Web service was the performance bottleneck with Reporting Services 2005.