The PSR-800 is great if you never have to put in a custom frequency, channel, or system - either "on the fly" or with the provided EZ-Scan Digital (which is anything but easy to use). I would rather stab myself in the ear with an ice pick than try to program a large trunk system - or any other system - by hand, with EZ-Scan Digital, ever again. You can't type information in quickly (cursor keys are disabled for moving from cell to cell) and the import from text files/clipboard is only partially available, and poor at that.
The user interface for the 800 is atrocious too. You have cursor keys and menus, and cascading menus, and more menus. Want to temporarily lock out one site of a trunk system (or even temporarily hold on one site)? Too bad. Go back to your computer, download the data to EZ-Scan Digital, make the changes, and reupload it.
The 436 is the most sensitive radio I have had since my PRO-43. Connected via multicoupler to the same antenna as a HomePatrol-1, a PSR-600, and a 996XT, the 436 is receiving things clearly that the HP1 barely picks up, and the other two don't even open up on. The digital decode is leaps and bounds over the HP1. The recordings are nice and clean. (I never got my PSR800 to record decently, and the playback interface in EZ-Scan Digital is, you guessed it, difficult to use and buggy as hell.) I do subtract points from the 436 for not being able to take the metadata off the audio files and display it on my PC, but it's just going to take somebody to write some software to do that, and there are people already working on it. The 436 also gets some critique from me for not having trunk analysis functioning yet, but that is supposed to be coming soon too. The GRE radios never had on-screen analysis, but I will admit my PSR-500 is the go-to radio when I need to do a Unitrunker session. If only it wasn't so deaf (and got even worse when the USB was connected).
The biggest, most important piece of info, though, is something you haven't given - where you are and what you listen to. That's what depends the most. The people who say the 436 is a giant steaming pile of crap are wrong. So are the people who say the 800 is garbage. There are people who holler and whine about how the Uniden radios prior to the x36 line are useless because they don't do P25 Phase II or don't do well on simulcast systems. Couldn't matter less to me; my area has neither of those. There are people who trash talk the GRE line for multiple reasons. For example I criticize the PSR-500/600 for only allowing 32 freqs per trunk system. Those radios don't do multi-site trunking at all. They also don't display information properly for P25 VHF systems. But if you have one of those radios and only have an 8-channel EDACS system in your area, you'll never come across those "problems".
In my opinion, if you have the money to spend, you can't go too wrong getting an x36, be it the 436 or 536. I've only had mine since Tuesday and have already fallen in love with it - just for the increased range over ALL my other radios, and the impressively improved digital decode. If I had the cash, I'd get about half-a-dozen 536s to put in my home office, my vehicle, my work office, the firehall, etc. I feel the radios are that good.
Again, what kind of stuff you're going to be listening to is going to determine if the radios are going to be worth your while (and $).