AHH MORE TEXT.
Bowser's dair is unique... By unique, I mean, you either love it or hate it; there is no in between, and there is good reason.
First, let's look at why people hate it.
-Terrible Landing Lag
-Takes Long To Finish (if attempted to be used in air for edgeguarding (wat))
-Damage is okay, but not too great for the risk
The most prominent effect is the landing lag. It's just terrible, and leaves you open for quite a while.
However, the dair is far from uselss; though situational at best. First of all, the damage is okay; around... 20 or so per hit, if they all connect that is.
The landing lag can be dealt with; at the end of the dair's hitbox (rather, when you land while performing it), a small win effect is experienced. It pushes away anyone hit by it, making the lag on the move almost null. Thus, a viable option unshielded.
In the air, Dair is good as well. In the air, the ending animation has a spike hidden when it ends. If all hits connect, the final one will send them spiraling down to their doom. Though it's very hard to pull off, a good set up is to short hop and immediately dair off stage; this allows you to recover from it, so long as you don't fastfall it.
On use I've noticed a certain Bowser use is when on the ledge, ledgehop dair back onto the stage iff they're trying to recover as well. The hits will connect, and the final one will push back, giving breathing room.
An interesting thing to note about dair is the shield stun; when you hit someone's shield, they will have to shield all of the hits, or else be subject to hit. And when the final hit connects, it will push back their shield to far and prevent a shield grab. It also shield pokes many characters, namely Gdubs and Honkey Kong.
Basically, dair is a situational move, but very good at what it does; damage building. 20 damage per execution is nice, and if you get that spike once (like I did) you'll have a sense of realization.
Hell yeah.