I did some research and decided the cost wasn't worth it since I can mulch most leaves.
Pros/cons I saw were
Exmark's ultravac adds very little length to mower in terms of parking it on a trailer. (you can remove bags and tuck another mower up to the back of this machine)
The bags allow you to dump into a truck bed w/o it being too much weight.
The ultravac is quick to attach/detach and convert to discharging. This allows one pass to reduce leaves on lawn and mow, then a final pass to vac up leaves.
The con was that you cannot use the unit with an extension hose to vaccum planting beds, leaf traps, curbs, etc.
Very quick to dump on property or onto a tarp in a trailer if you have access to it.
Lower capacity in many cases than bags.
Cannot dump it into truck bed or other raised container w/o double handling debris
Cans (trac vac type system)
-Takes up space on back of machine, lengthening it so that you may not be able to fit another mower on the trailer
-Added weight/complexity of an engine
-lower capacity in some versions
-Trailering type are really long
-Big plus is ability to use it as a leaf vaccum with accessory hose if needed.
-Possibly may not work as well as a system custom-designed to work with a specific mower.
Be sure to check total width of unit. Side mounted engines or impellers can make some mowers wider than your trailer or they barely squeeze on.
It's no help now, but you will find used units in good shape for about half the price after the leaf season.
They apparently do not reduce leaves as much as one might hope, so for heavy accumulations a first-pass over the lawn to reduce volume might be in order. For that reason units that detach easily come in handy.
-affects weight distribution
I'd lean towards a unit supported by a good dealer in case you need parts quickly. A lot of the aftermarket units I checked had spotty dealer networks (would walk in to where they said they were sold and got puzzled looks when I asked about them).
They are really overpriced for what you're getting. Except that if you need one you really need it! $2,000 for some moulded plastic and tubular framing like you'd find on a lawn chair costing a few hundred, 3 bags, a pulley, a belt, and an impeller. It's a low volume item and those always cost more.