The E190 is considered mainline at JetBlue and most other carriers, but its little brother the E170 is in that fuzzy area in between regional jet and mainline equipment.
Having flown on the E190 a few times, I have to say that it is NO regional jet. And no DC-9, for that matter. (The DC-9 is typically a 2-3 seating config in coach; the E170/E190 is 2-2). The DC-9 was a fine machine in its time, but that time was 30-40 years ago and the torch has passed since then. Although there have been well-documented reliability problems with the large Embraers - they are a relatively new aircraft type - the passenger experience on-board is very good. I'm a very big fan of the large windows and comfortable seats. In my opinion, the E190 seat comfort is as good or better than any other narrowbody equipment (in coach, anyway), and there is no comparison to your average regional jet. JetBlue's comfort level is enhanced by the tv and XM
radio, which are great devices for killing the time between takeoff and landing. Or while stuck on the ground waiting for takeoff (which can sometimes be longer than the average flight on the E190!).
Remember that Boeing pulled the plug on the 717 prior to the arrival of the E170 and E190 families. The 717 is a much bigger, heavier aircraft than the E190, and can hold more passengers, yet I believe the E190 has a greater range. With only 15 more seats than a CRJ-900 series regional jet, more and more airlines are shying away from the Greyhound bus seating arrangement of the CRJs and the E170/190 makes a great alternative. I am not knocking the DC-9/MD-80/90/B717 models. They are great aircraft, and the B717 was a much different plane that the first DC-9 that rolled out of Long Beach in the 1960s. But I'd suggest that Boeing killed this model because it potentially interfered with their own 737 models, not really the new wave of 100-seat aircraft such as the E190/195 (and eventually its Canadair challenger).
Considering how much changed in the 40 years of aviation prior to the DC-9 arriving on the scene, it is easy to think that technology has not come very far in the 40 years since. Indeed, we are still flying subsonically in similar seating arrangements, and some would even say that the level of personal service has diminished greatly since the early days of jet-powered aircraft. But this would be oversimplifying things. Aircraft today are much more technologically advanced, with lighter materials, safer and more efficient engines, etc. The E190 is a great example of a light, economical, comfortable airplane, that has the legs to fly 2/3 across the USA.