Check-in: It can be a mess with hostmaker (agency host uses) as someone else noted here as well. The keys were incorrect, the sets of keys they use don't all work for all locks. I ended up with five sets. Hostmaker tried their best to help me and waived the cleaning fee due to inconvenience. However, I was talking to two people who weren't in sync and I had to constantly call back to check on status while I was stranded outside for 2.5 hours. There was no in-person intervention through the process.
Place: Its nice, clean, convenient, a bright living room, and we experienced how it is to live in London as we were working through the week.
Location: Very convenient to Marylebone station and 15 min walk to the center of Mayfair/Marylebone.
Would I recommend? - I'd if the host fixes their key sets, check-in process, and communication. This agency thing isn't working.