Sketchbook is definitely not for everyone. It is one of those slice-of-life 4-komas that rely on everyday situations and likable characters. If you're looking for action or comedy, then Sketchbook is not for you. There are funny moments, certainly, but the majority of funny moments consist of the reader going, "Oh man, I've done the exact same thing." Not really a laugh-out-loud moment, but more of a chuckle-at-self moment.
On that note, I do believe that Sketchbook succeeds at what it sets out to do. It has an enormous cast, and it doesn't shirk away from introducing new characters even 100 chapters in. And, while, to be honest, I don't remember everyone's name, I do remember all of their faces and personalities. They're all likable and unique enough to be memorable, all the while not being too over-the-top.
Overall, in terms of slice-of-life, I find this manga the most relatable, and it is definitely one of my personal favorites. Nothing exciting really ever happens, but I think that's part of the beauty of Sketchbook.