Funny you mention this. I stayed there recently, but literally can't report on much. I was attending the FT Uni & stayed a couple of nights at LGA Sheraton (smack dab in the heart of Chinatown I think; might be Koreatown - I'm not up on Asian language or food). Marriott not hugely far away from Sheraton, but smack dab next to airport.
The hotel is certainly serviceable for a stay when you have an early flight out of LGA, and as you said clean, etc.
I checked in late aft in a hurry cuz other FTers were waiting in car service to go into Manhattan for a going away party for an FTer.
It was a Sat. The bellman & FD were busy (I think due to a wedding or something). The check-in was perfunctuary at best, but I think that was specific to the agent who didn't appear to give two patooties. I was a bit cranky at her who gives a you know what attitude, but I was in a hurry.
I was NOT happy as a Plat to be located literally behind the elevators, and had I had more time probably would have gone down & requested a room change, if not an upgrade, but at least a room change. But as mentioned I was in a hurry.
The cab driver (this has nothing to do w/ Marriott) was a nimrod on the way back from Manhattan & first took me to the Courtyard. I made him turn off the meter at that point & 2 more hotel stops later I finally spotted the Marriott sign & got delivered to the right place. So someone might want to be very specific about which property since Courtyard by Marriott & Marriott seem interchangable to these guys if they're coming from Manhattan.
When I walked in some of the airline crews were checking in, and it seemed there were a few weddings or parties going on (this was a Sat night - not sure it's like that all the time, but a reference nonetheless).
Weirdly enough the room fan kept me up/was noisy, so I finally got up & shut the fan off on the wall switch & that made it easier. I didn't realize it wasn't planes until I turned the fan off.
I got mixed answers on how early the shuttle started in the a.m. (I had a 6am flight), but basically from 4:45am on you're good to go. Might be earlier, but that was the one I took. It's just a couple of minutes away.
BTW - I had a bit of a hiccup w/ the bill, and the overnight auditor was there at the front desk & sorted it. He had a pile of newspapers in front of him, which evidently were just delivered, so that was pretty funny. And the FD crew at oh-dawn-hundred were much nicer than the aft person.
Long story short - I'd stay here again if I needed to be in the area/had an early morning flight, etc.
Cheers.