I've flown Allegiant 4 times and due to its changing and testing airport markets, you can't bank on being able to fly out of "XYZ" airport 14 months from now. But you can generally plan on being able to fly up until the point that tickets are sold online. (I understand that Allentown being shutdown earlier is an exception to this, but I don't think that is usual). I think that's also a reason for why Allegiant doesn't put their travel dates up as far out as other airlines do.
Having said that, Allegiant's planes are great. Service is always fine. All my flights have always been on time. And twice, I've flown Round-Trip with Children (either 1 or 2yrs old and 4 or 5yrs old at the time). And I've just booked my third trip with children. I've flown twice roundtrip for business by myself.
I've never bought reserved seats, but maybe that's because our route isn't generally full. I've never had problems sitting with my children.
You can look at what the seat availability is like...if it's really full...then pay the $11 each way; if not, then don't. Now if you're booking really early (i.e. 3-4 months in advance, then this advice might not be practical). Which brings up another point.
Allegiant is great for last minute trips, because generally they lower their prices the closer you get to the travel date. Again, if you're flying on a particular popular route, this might not be the case.
But for example, I just booked 4 tickets for early February at $29 to SFB and $29 back. You can't beat that; no matter if you add in luggage costs, fees, and reserved seating. Just a week ago, one of those fares was $74. But I checked the seating (and saw that there were still plenty of seats available on the plane--only 26 were sold), so I opted to wait and BINGO...the price dropped and I scooped up 4 tickets. So 4 of us fly for $340 ($85 each Roundtrip) total round trip including fees, 4 pieces of luggage total (no reserved seats)....NON-STOP.
I booked a full-size car through
Alamo for a week and after a $25 off weekly rental coupon code and the
Walmart Code, my full-size car is $124.99 (equivalent to $17.86/day); granted the price increases with fees and everything, but still, my rental for a week in a full-size (Pontiac Grand Prix or similar) is $173.01.
We then have the option if we need to run to a Walgreens, or want to go to Sea World, or even if we just want to drive to a WDW park. I love the flexibility. And I don't have to wait on DME or get my bags ready by the crack of dawn, so for me, I love the flexibility of having a rental car.