I have a few applications now and I dog food some of them and others its not really possible.

I think when specing out a system I'll try if at all possible to have a go at working through some of the work of the clients to get a feel of what they are needing in a try someone else's dog food scenario. I think this is important to do rather than a pure tell me what you want meeting.

Frequently they have no understanding of the table structure of their existing systems and it can be extremely useful to sit over the shoulder of an in house super user and just let them take you through their daily work and let them describe their frustrations and the points they consider good.

But generally I would be a massive fan of dog fooding

I would say that it is a massive advantage to be a developer and user of software. If you really believe your software gives you an advantage in your day to day business then using your own software should be a given.