I've been running what I personally call 'SEA-style' (mostly popular in China and Singapore; can't find them here in Thailand) OH trickle filters on my 2 X 325 gallon Asian Arowana tanks for about 8 months now, thought I'd share some pro & cons.
-- Overall quite effective filters, esp. with regards to making a nice, oxygen-rich home for beneficial bacteria. That said I wouldn't use them as sole filtration on a bigger tank; imo they work best as supplementary bio-filtration.
-- Reasonably Flexible: you can change media or add tray levels as needed pretty easily (though one wouldn't want to go too high with them).
-- Esp. in small-medium size tanks you really don't need a big pump. In my case though I'm using bigger (8,000 & 12,000 l/hr) DC pumps because they're also feeding external UV sterilizers and there's also a decent amount of head pressure in the system.
-- For me, with my relatively low bio-loads and bare bottom tanks that are siphoned at least once a day, they are quite low-maintenance, I only need to clean the course pads (and change the fine pads) about every 1.5 months. For those with heavy bio-loads it would be different but that's true of every type of filter.
-- They're quite safe and easy to use compared either canisters or sumps. Really the only thing that could go wrong (besides falling over during an earthquake or tsunami) is that a hose could slip off a connector but that's easily solved with hose clamps. These units are great for non-mechanical/engineering types like myself.
--From the way the unit is designed (with only one gravity fed outlet hole), you can only get so much flow going through them -- certainly less than canister filters or sumps -- so turn over rate could be an issue on larger tanks, which is why imo they're better as supplementary filtration. That said I guess you could drill another outlet hole at the other end if you wanted to allow more flow.
--If you have a high bio-load or say planted tank with good amount of leaves or detritus, you'll have to be cleaning/changing pads probably at least weekly (though a good pre-filter on the intake would help, as both of my pumps have).
-- If water supply to the trays stops (for cleaning pads, changing media or pump, or during no-power situations, the main PIA is that you have to manually pour water into the tray tops about every 10 min. or so to keep the media from drying out and killing your BB. If you weren't home or didn't have auto electric back-up, you could lose most if not maybe all the BB pretty easily. If same happened with a canister or sump, I think the still submerged bio-media/BB would have a higher survival rate.
-- As they come in the box they are pretty noisy due to the return flow falling into the tank, like any OHF. However, I got around this very easily by installing a common sink drainage hose as pictured below. You can barely hear the water coming down and this type is adjustable in length or angle.
-- Looks: I really don't mind them at all but some might not like the aesthetics of having it on top of the tank and so visible.
Please excuse my as usual long writing style but hope the info helps at least some. You can find these on Amazon.