As above anyone use one. I'm not new to bikes but this will be the first time I've carried stuff on my bike and not my back

Not used one (so really I shouldn't be here), however, just interested in your reasoning behind one rather than soft luggage - genuine interest.

Just to stop the swing. I've had a quick look and it's not much heavier than a harness. I don't have any experience with the swinging seat pack and don't know if it's really an issue

I don't have any experience with the swinging seat pack and don't know if it's really an issue

Some are better than others and much is to do with (a) physical size (b) how well they're packed. However, even the biggest, flappiest ones are never really an issue and any swinging is largely only noticed when climbing while stood. I do wonder whether it might be a 'problem' that's been blown out of proportion somewhere along the line?

There shouldn't be much "swing" in a seat pack, if there is then you've either not attached it correctly or you've got too much (or too heavy) stuff in it. It could also be that the attachment point to the saddle rails isn't up to the job. IME seat harnesses like the Wildcat Tiger are pretty solid, they do move a bit but you really don't notice it when riding.