I got into the show last year when my girlfriend introdouced it to me, and now it's a pretty big thing amoung my group of friends.
I also just finished reading the books, A Song of Ice and Fire, which the show is based off of, about a month or two ago. So this season will be the first season I watch (and probably the last considering the next season will probably come out before the next book) where I know what should be happening. I also just finished rewatching the first four seasons about a week ago in anticipation of the new season.
I expect there will be a lot of changes this season, but at this point I'd honestly rather they go off the rails diverging to avoid spoiling content from the next two books
As for the first episode of the season that priemered yesterday, I thought it was solid. I'm not sure if it's just because I've read the books, but for a first episode, I felt like it didn't really show the aftermath of Tywin's death as well as the opener to season 4 showed the aftermath to the Red Wedding. Rather than opening up with the prophecy (or still doing that but following with this scene) I feel like it would have been more impactful to start with the scene immediately after Tyrion leaves and everyone in the Red Keep panicing over Tywin's death. Cersai being woken out of her bed in a panic to find out her father was dead would have worked better than going straight to the funeral. Would have worked as a way to keep the energy high for the first episode, and as a reminder to people that Tywin's death is a big deal.
I enjoyed the episode overall though. I thought the Unsullied/Sons of the Harpy scene was well done, it was good to see Brienne so soon, and I enjoyed seeing Kevan and Lancel returning to the show. The wall scenes were all good, I think they're completely skipping
offering Jon Winterfell and trading his Snow name for a true Stark name
, and I'm not sure what the plan will be with Davos at the wall.
You can't expect much from a first episode as they have a lot of catching up to do, but I thought it was a fine start.
EDIT: As for spoilers, I'd say everything from seasons 1-4 is fair game, as well as all officially released episodes. Spoilers for future episodes and book events should use spoiler tags.