Hi Dean,
I use a Backpack ALL the time. In fact I now employ a chap and have 2 Backpacks

Now there are some on here (Groundhog :

:

) who will tell you a backpack is useless. NOT SO!
Depends on what type of work you do.
Domestic and small commercial (like my round) a BP if just the job

Large Commercial Contracting. A bit like mowing a football pitch with a push mower
ADVANTAGESGrab n GO ANYWHERE

No 100mtrs of hose to reel up and drag around
DISADVANTAGESGetting it on
Filling up more often
Now these disadvantages can be overcome.
If you put half a dozen 25ltr barrels, a backpack and a pole in your car theoretically you could be of WFP. But imagine; humping a full 25ltr barrel to fill up the BP THEN humping a full BP up and onto your back. Been there, done that, NO NO NO.
So I designed a Trailer system where the BP is mounted on the back at back height and a fast transfer pump in the tank to fill with.
There are a few other questions I'll cover later