Like many hotels in Brighton, the Mercure is fairly old, and you've got to expect it to have some quirks and oddities. Similarly, I was only staying one night, so I wasn't particularly bothered by the oddities as such.
However, there are a few things that stick in my mind.
I would have liked a sea view (kind of a selling point for a seafront hotel). Alas not. Alarm bells probably should have rung when I got a free upgrade to a double room. I'm curious about what my lower grade room would have been if this was the upgrade.
I was in Room 115, which faces the rear lightwell, and it has no real view to speak of, unless you like escape stairs, and seagull excrement. Not lovely.
The bathroom layout is very narrow and linear. If you're a larger person, avoid this room. The shower was a 700 x 700 cubicle. I accidentally turned the shower off when I was turning around, and I'm not a large person. Similarly, I had a to turn sideways to get past the washbasin to reach the toilet - my hips jammed against the wall otherwise. The toilet is on a step up from the main floor, and slightly hard to reach.
Overall, the staff were very nice, the food was good (I only had breakfast). The breakfast room is slightly awkwardly laid out, and slightly cramped but it was fine and they've made a decent go at it.
Overall, it was fine for a night and my visit was generally good. But this room really needs retiring - it's not great.