Matrix downloads, from my past experience, have been trouble free. I have bought, I think, about 8-10 titles with digital download from Matrix.
On the flip side, some people have had issues, but from what I gather they have been few and far between.
When you go through the payment process, you will be taken to a download link. Clicking on the download button will download the software to the drive. Around the same time you will receive an email which will have your order details on it AND a serial key. When you open the downloaded EXE file to install, you will be asked for the serial key. Simply highlight it in the email, copy it and paste it into the box. No need to be online. No limited installs. As long as you keep your serial number safe (which is unique to you) you can even download again and again.
After installation, burn the downloaded exe file and serial number (in a text file) to the DVD for safe keeping.
I AM the Law!