I've not answered this as I've never had cause to use our Staff Travel scheme, and I was hoping someone who had could give you the info you want. However...
You are entitled to use the Staff Travel scheme once you've been in the company for 6 months. You may nominate up to 14 (I think) companions who may travel with you on various concessionary fares. 1 companion can be nominated to travel unaccompanied on concessionary rates. The number of concessionary fares you are allowed depends on your seniority and your job grade. Most concessionary fares are on a standby basis,but 'Mates' and 'Mates Plus' discounted tickets are firm. After 5 years with the company, you are allowed a Priority Annual Trip (or something like that) which allows you and your family to travel to and from a destination at the highest priority standby. You are also entitled to a significant discount on Virgin Holidays accommodation or packages on these trips. I understand that these Priority Annual flights are usually (but not always) converted to firm seats before departure, where possible.
As I say, the majority of staff travel is on a standby basis, so it pays to pick your trips carefully. Attempting to get to the Caribbean in winter, Orlando in school holidays, or Hong Kong for Chinese New Year is likely to be unsuccessful! There are, I'm sure, other aspects to staff travel, but as I've never used it, I can't tell you. Whether it's good compared to other airlines, again, I can't say. Hopefully someone with more knowledge can help out.