We did a Universal-only trip in February (5 nights, so four full park days) and it was awesome. It was relaxing and fun at the same time. No stress. I still like the Disney parks a bit more, but as a whole experience, Universal-onsite blows Disney away. Hotel was better, food was better, and no planning was amazing.
I'm kind of glad we did it that way for the first time instead of trying to mix Disney and Uni. I think because we committed to a decent length of stay at UOR and all onsite, we really gave Universal a chance to grow on us, and it did. Sometimes I think people who just visit for a day don't get the full experience and then compare it to the full experience at WDW, which is kind of an unfair comparison.
Our next trip is in August and we are splitting the trip, half onsite at UOR and then half offsite and we will see Disney from offsite. So that will be a nice mix too. But I'm still glad we took that Universal-only trip!