My experience
I have done a number of performance and load tests, both in projects where I was the test leader and in ”stand alone” performance test assignments. In the tests I have used various tools. For simple tests on web applications that do not use https I most often have used Open STA. In other cases I have used Mercury Loadrunner and Webload. For testing of webservices I have used SoapUI. This is a very good tool and I to be honest I have mostly used it for performing functional tests on SOAP webservices. In my tests of for example FTP downloads and java beans I have used JMeter.

Expensive tools
I think that what tool you use it not that important. What is most important is that you know what to do, how to set up the test, how your test scripts should be like etc. However, it cannot be neglected that a good tool (often an expensive tool) makes it much easier and quicker to carry out and analyse the tests. Also, since it is easier to use it is easier to produce scripts that are similar to the actual load the application will meet when set in production. Learn how to load test .Net: Performance Testing Microsoft .NET Web Applications. General knowledge of load testing web applications can be found in this book: Integrated Approach to Web Performance Testing: A Practitioner's Guide

Start early
A very important thing is to start load testing early. In the early stages of development you should pinpoint the functions / parts of the application where good performance is vital. It could be a database connection, a webservice that will be frequently used etc. By load testing those functions very early you will save a lot of trouble later on in the project.