So far I'm level 22, just doing the main missions and some of the side stuff with a bud. Haven't even touched Dork Zone content yet and can say that I think this is worth the dosh I dropped for it, for the enjoyment I get out of it and the time I'll have sunk into it by the time I hit 30 and have played through the main missions.
Yeah, the side missions and resource missions are definitely repetitive in terms of format, but are still for their part fun and fairly challenging alone or with friends. Doing the main missions on hard opens up some replay value (I had to wait until level 20 before I could tackle the first mission, level 4, on hard- and then we wiped a few times haha) which is nice. The controls and cover system are also pretty top tier IMO, and the customization aspect (focusing on firearms, or support, or tech/turrets/bombs or some combination- or just in aesthetics, clothing, etc) is pretty rad, a combination of Destiny and Diablo 3(in terms of re-rolling stats especially).
Can't really speak for "end-game" content and replay value yet, but the way up to it has been great, very few complaints (weird bugs where I get stuck in non-game areas, mostly).