Is Montersat a "must see" when visiting Barcelona and what's the best plan if we decide to go? Find a day tour at one of the TIs?
This is a tough call; to go as a "tourist," I would think it would be kind of a so-so tour. As a believer, it is a pretty amazing experience. I went with a person who really wasn't that into "religion stuff" and he was deeply impressed by the piety and respect that he saw from the locals and especially the little kids brought there by the nns and church groups. Hope this helps.
We enjoyed our trip there (which we did on our own), but I wouldn't consider it a "must-see." It depends on how much time you have in Barcelona, which in itself is wonderful. We thought the very best thing would be to spend the night at Montserrat, because it was pretty full during the day.
If you have the time to fit it into your schedule then I would certainly visit Montserrat. We were a family group of 8 that went in December - just went on our own, not as part of a tour. We all thought it was beautiful and were happy we went. We took the train from Barcelona and then two of us went up on the cog and the others went up on the gondola. Easy enough to do. The entrance to the church is particularly impressive. We spent several hours up there and came down on the last cog of the day. If you are not comfortable navigating the trains by yourself your hotel or the TI can guide you to available tours. Don't know how much time you have for Barcelona but there are lots of great day trips easily done by train and the train fares are cheap. We also enjoyed a day trip up to Girona.
I liked visiting. Unfortunately, we were hoping for some hiking but it was cold and starting to rain, so we only visited the monestary. We got there before the crowds and enjoyed our brief visit. The gift shop was thousands of square feet of Tan Virgens, big and small, which we found amusing (we're Christian but not Catholic).