The most important thing about Semrail was that they were able to show us how our website could perform under many different scenarios by using very realistic traffic figures for each scenario (i didn't realise this at first).
This made it easy as i knew what would happen if my site got more or less visitors! There are other tools around which will do some of these tests but not all - so far sem rail has been good with showing me where I can improve things without breaking anything too badly!! They have also helped implement new features such as custom landing page creation etc., although their support team aren’t always quite up there yet when you need them quickly.