If you went with your girlfriend to watch a romantic movie and you do not really like it. Afterwards, you could say" The movie was ok but it was not my cup of tea." I It is a personal opinion, mostly used in the first person but also could be used expressing an opinion that someone else did not like something

Firstly, you use it only as "It's not my cup of tea". People don't really say "It is my cup of tea". You use when talk about tastes or preferences. Like, diving is not my cup of tea. Means that I not really like it.

Hi Moisés, Well, I'm sure you know that the phrase means "I don't really like it (ie. usually a certain activity), but maybe other people would." In this case, you do need to say whose cup of tea. "A cup of tea" could mean any possible cup of tea... even a literal cup. Yes, you can change the possession of "the cup": "It's not his cup of tea", for example. I noticed a couple of members simply crossed out the sentence. This is probably because we couldn't tell what you were actually referring to. Perhaps you meant cycling, but you had already begun talking about cycle lanes before you added the idiom. It just seemed a very out-of-place thing to add. I'd have expected to see the expression in the middle of the paragraph, not at the end.

"It was a hairy project for them" sounds good or not? I'd say the equivalent in Spanish is "peliagudo". It means very difficult, very hard "Fue un proyecto peliagudo".

There is a colloquial expression "to be hairy", which means that a situation is slightly dangerous or uncomfortable. So you could say "cycling in the city can be a little bit hairy", or "the taxi ride to the airport was quite hairy" or "the fans started to throw stones and the the situation started to get hairy".