Ditto to what Rickair said (Brand X...(cough, cough "Mesa" )

For instance, most companies (such as SkyWest) have preventative MX. So, they're constantly being proactive to MX issues, versus a place like Mesa, which is reactive. HUGE difference between the two. SkyWest will attempt to fix the issue before it happens, versus Mesa, which waits for the issue to happen...then tries to fix it.

As Rickair pointed out, APU's are a prime example. I can look back on a 4-day trip and practically guarantee that, of the 7 aircraft I flew during that trip, roughly half had either no APU, or were running on a single pack (which is about the same in terms of discomfort). On the other hand, SkyWest (or any other carrier, for that matter) can look back on the same trip, having swapped the same amount of aircraft, and had fully functioning APU's and packs throughout the entirety. Both SkyWest and Mesa have the same APU's in their CRJ's, but SkyWest proactively deals with MX, while Mesa waits for the issue to become a problem.

For example, let's take a SkyWest plane, and a Mesa plane, both scheduled to depart ORD at 1300 on a sweltering 97 degree day.

1230 pm:
- SkyWest pax board a plane that's extremely cool and comfortable.
- Mesa pax board a silent airplane (meaning no APU) with cabin temp's already close to 100 degrees.

1253 pm:
- The SkyWest airplane pushes off the gate seven minutes early.
- Mesa crews, for the fourth time, are calling Op's for an air cart and GPU.

1335 pm:
- SkyWest is climbing through FL190...FA calls up the crew and says "hey guys, can you turn the AC down a bit, it's a little cool back here".
- The Mesa plane finally gets clearance for pushback, and cabin temps are at 103 degrees.

1347 pm:
- Currently number nine for departure, the Mesa FA calls up the crew and states that a lady in the back is dizzy and throwing-up due to the excessive heat.

1410 pm:
- The Mesa crew returns to the gate, deplanes the furious pax, and calls MX in attempts to fix the APU. MX is already fixing another faulty APU on a seperate airplane, and states that they can't get to their airplane for another hour.

1520 pm:
- Skywest arrives at their destination 13 minutes early
- Mesa cancels flight.

So, in answer to the original question...yes, pax do notice a difference in quality.