Loved it. Rogue One is a completely different emotional theme than Force Awakens, and it should be. Rogue One begins a little slow, but by the end, i completely forgot about that. The last 5-10 minutes really felt like an emotional gut punch, and were incredibly well done. i went into this thinking i knew what would happen to most of the characters, and it still hit me hard.
I really don't see any of the cameos as fan service, except for the dudes from the cantina...THAT'S fan service - having those dudes be on a completely different planet days before their appearance on Tantooine in ANH, and they just happen to be there on Jeddah.
R2 and C3PO needed to be there, or else how would they have been on the ship with Leia 5 minutes after Rogue One is over? Leia NEEDED to be there. In ANH she has the plans that she just got from the rebels who stole them - Rogue One is a movie about stealing those plans. Tarkin NEEDED to be there. In ANH he is in command of the Death Star. So he needed to be in place there since this is the same timeline. The X-Wing pilots needed to be there too, since they are part of the rebel fleet at that time and this was an all out attack with the entire rebel fleet.
I will say this - this wasn't a film for casual SW fans. That seems like a specific decision that was made. It's definitely not for kids the same way all the saga films are, and I would imagine that it is confusing for someone who just watches the main films and doesn't watch the TV shows or read comics. It's a bold move - because that is definitely gonna hurt it in the box office compared to The Force Awakens or the Marvel movies. BUT I think it makes for a better film.
I think it will age well. This is the first time that comparisons to Empire Strike Back seem valid. ESB definitely didn't have as much mainstream appeal as ANH when it dropped. I remember as a little kid not liking it as much because it didn't end with the good guys "winning" or a big space battle like ANH. But by the time I was a teenage ESB was clearly my favorite.