Mike,
I use the Devault plant dolly also, I like the 16" the best, the 24" is too big unless you are using half barrels. I bought one 24" and use it to put multiple small plants on in the sun room. I buy a few 16" each year and they are very heavy duty. I keep some of my bigger pots(12 gal terracotta) on them year round with no sign of impending failure yet. I just put a 28 gal terracotta pot on one and so far it is handling the weight fine.
I like them because they make the shuffle easy, and I can put my pots wherever I want in the garage. When I need to get to something behind them or need to do some organizing they simply roll wherever with simple guidance. Same goes for the wife, if she needs to get into the paint cabinet it's no problem for her to move a 8 or 12 gallon terracotta pot. I agree 100% on price justification, compared to back or herinia problems the price is nothing. I also shuffle my plants from the deck in and out of our sunroom in the fall so again for the bigger pots it's nice to have them very mobile.
Its not a whole lot, but HomeDepot.com has the 16" dolly for $2 cheaper each and when you spend something like $45, the shipping is free; that is why I buy 3 at a time. Also the company that ships them is lightning fast and I usually have them 3 days after I ordered, standard shipping.
I do have to add though, I will also be buying a hand truck for the trees that go on the other side of the yard. Unless you want to shuffle 2 plywood sheets, dollies are useless on surfaces that aren't relatively flat and solid.
One more thing, for anyone thinking about buying a cheaper plant dolly somewhere. I mentioned more than once that Devault are heavy duty. There might be others out there that are just as good, but so far all the other plant dollies I have inspected are garbage in comparison to Devault and are going to top out around 20-30 lbs and the wheels and or frames will likely fail early. IMO.