It's completely different for every relationship. Some poly people in primary partnerships find that they derive a lot of side benefits -- better communication, better trust, better sex -- from having OSOs. Others find the EXACT opposite, and find that everything is harder. Yet others find that there's relatively little impact.
In the end, though, what matters is how *you* are treated by your partner, and how satisfied you are with this particular relationship.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.