As someone who is fairly well locked-in to both the DC and Marvel lines, I usually read the big line-wide events. I even sat through Fear Itself. I tend to skrimp on tie-ins because I find most of them to be pointless cash grabs, but most of the time I get the main series.
Until Infinity rolled around. I have not been reading Infinity. Having lost interest in Hickman's Avengers awhile back, and having less than zero interest in Marvel's space characters and/ or the Inhumans, this one just didn't look like something I'd enjoy. So I sat it out. The series did, however, cross over into several books I do read. It also launched a new series that seemed appealing, and it tied into another series in a way that coincided with a creative change I found interesting. Much has been made of the notion that Infinity is a series where the tie-ins really are important, so much so that the main series reportedly is incomprehensible without also reading Hickman's other Avengers titles. My situation got me curious, though - turn the "are the tie-ins key" question around - how do regular series that tie into an event hold up for someone who is not reading the event? In other words, are Marvel and DC's tentpole event books required reading for people heavily invested in their respective universes?
Reading list: Infinity crossed into Avengers Assemble (#18-20), New Avengers (# 9-11 so far), Superior Spider-Man Team-Up (#3-4), Mighty Avengers (# 1-2 so far), Secret Avengers (#10 so far), and Thunderbolts (#14).
Criteria: It seems to me that the things important to me are, in no particular order, that (a) the issues of books I read are by the usual creative team, (b) they are comprehensible without having read the main series, and (c) of course, they are good reads. So here's how it shook out.
Avengers Assemble # 18-20: The first two issues were written by Kelly Sue Deconnick, who is the regular writer, while # 20 was written by Al Ewing, who has filled in previously. #18 and #19 are set very clearly in-between panels of the main series, but the learning curve is pretty slight. The biggest problem with them is that, in addition to being tied into Infinity, they also seem to be tied into whatever is happening in Captain Marvel, which I do not read. #18 works fine but then there's some disconnect when #19 doesn't follow. #20 was a total fill-in featuring completely different characters but worked really well as a standalone, and had basically no learning curve at all.
New Avengers # 9-11: As mentioned, I lost interest in Avengers but had still been reading and enjoying New Avengers. Unfortunately these issues lost me. They're by Hickman, who is the regular writer, but are tightly woven into the Infinity story. Almost completely incomprehensible on their own. Enough so that I doubt I will keep getting the title, even when the event is over, because inevitably there will be another one. Increasingly it seems one has to either go all-in on Hickman's Avengers books or skip them entirely.
Superior Spider-Man Team-Up # 3-4: These issues were not by the regular creative team and as such I did not get them. Like its predecessor Avenging Spider-Man, SSMTU is set up in a way such that fill-ins are relatively easy to slot. The idea of two fill-in issues that are also event tie-ins just didn't appeal to me, though. (To be fair, Avengers Assemble #20 was the same thing but I got it anyway because I really enjoy Al Ewing's work.)
Mighty Avengers # 1-2: And Ewing writes this one which is why I got it. This storyline ties pretty tightly into Infinity but does a very good job of explaining everything the reader needs to know. The events of Infinity are more a backdrop for this story rather than necessary reading. I never felt lost with these issues; nor did I feel like they were servicing an event rather than developing their own story. This is IMO the best way to tie in, if tie in one must.
Secret Avengers #10: This was drawn by the regular artist, Luke Ross, but got a fill-in script from Ed Brisson. It's the first half of a story that continues in #11, and it has little to do with the ongoing storylines that have been featured in this series. It is completely comprehensible, though - basically all you need to know is shit has hit the fan and the Avengers are off-world. With a book like this, heavily built on espionage and using more street-level characters, there's a danger of sacrificing the book's usual tone in service to the crossover, but thankfully that doesn't seem to have occurred here.
Thunderbolts #14: This was a book I had not been reading, but it seemed to be the start of a new storyline, and Charles Soule recently came aboard as the writer, so I wanted to check it out. I did not enjoy the art, though I've liked the artist's work in the past, and did not continue past the one issue. It was only a loose tie-in, though - my issues with it had nothing to do with Infinity.
So end of the day, Infinity drove me off of one title I had been reading, caused me to skip a few issues of a second title, but launched a new series that I'm now following. Of course, Mighty Avengers didn't need to tie into Infinity to get my attention, but neither did the fact that it tied in work as an impediment to me picking it up. Presumably Marvel felt the book would launch better as part of an event, and they probably know more about that than I do, so whatever.
It's interesting to me how much things have changed over the years. As a kid I got most books via subscription, and I was pretty out of touch on events or tie-ins and stuff like that. It seems to me that it would be almost impossible to be that kind of fan these days, at least for anyone getting more than a few of the books. Even if I'm not interested in Infinity, I kind of need to know what it is just so I can keep track of the books I do read. I give Marvel a lot of credit, because usually they do a very good job of structuring their events. Even shitty events like Fear Itself usually are well-organized and relatively reader friendly. There's still a pretty high degree of required reader involvement for this stuff, though.