No, I don't really do combos that often. I prefer standard attacks and special moves when fighting.

Uh... HELL YES I USE THEM OFTEN!!!
Especially online, ALTHOUGH I do have an odd strategy. I will do as many dial-a-combos as I can in the beginning of the round, so that my opponent wastes all of their combo breakers.
The SECOND that their last combo breaker is used, I will IMMEDIATELY go to pop-up attacks, and do 15+ hit juggle combos that take WELL over 45% by the time you land. OR. . .
I will use a pop-up attack in the MIDDLE of the stage, and then, like I said, do a VERY large dial-a-combo, juggle them, and knock them RIGHT INTO A DEATH trap before they even touch the ground. So basically, once my opponent loses their combo breakers, guess what?
Their ass is mine. No exceptions. No mistakes. The ONLY thing that saves my opponents is combo breakers. And they don't do very much. Especially if they piss me off. Then, NOTHING can save them. I'll keep THAT strategy to myself.
So in answer to your question, yes, pop-up attacks are probably the most useful maneuvers in the entire game, besides the block button.

That is why I preserve combo breakers. I only use them when I'm on the offensive and cought in a combo that actually hits me.
I prefer the defensive play, but that depends. I call it volcano style, becouse of all of a sudden it suprises everyone. IN the matches I rarely use attacks, and avoid the enemies projectiles with sidestepping and blocking. I hurl projectiles a few times, maybe three or two, or do some charging attacks. -mostly with Hotaru-
When the opponents comes too slose I use pop ups and when they are cought off guard baddly, a 56-85% damage combo comes hard! Usually multi-style combos, and when I'm done with the majority of their health, I leave them to play with my harassment.
PRESERVE COMBO BREAKERS FOR GODS SAKE!

This is my strategy.
Pick Mileena, pop 'em in the air preform Outworld Wonder few time, pop em in the air break thru throws repeat to win.

wit lui kang jus do -----> triangle like 2 times square 2 times, and then if u pulled this off rite, do a bicycle kick, and speaking of bicycle kick, wat happened to "hold tha button for 5 seconds" or sumthin like that that was in tha mk klassics?

I actully like the new way better, I didn't like charging a button to do the bike kick

Holding buttons is not a good idea for controlls like PS or other consoles have. It is just the data flow that interrrupts when you do that, so no moving fluidly while you push a button -I suggest.

I'll only do pop-up attacks if i've successfully sidestepped around the opponents attack, when it is very hard for them to block in time. This is because pop up attacks usually leave your character vulnerable to attack for a significant amount of time, so they are dangerous to use when facing each other. Sometimes I will trip my opponent up with a low attack and as they are coming up do a pop up attack which catches them off guard.

Pretty much all of the best combos in the game use them. Don't overuse in a combo. And I mean that damage wise, sometimes two popups will lessen damage in the end. A poke that did 5% normally, after the second pop-up may do just a 1%. Play to win. If you want to know the amount of popups to use tho, check training. I fought a scorpion who used 2,3,up+4,up+4, 2,2,cs,1,3,3,cs,hellfire. leave out that second popup...it does more damage. Hell you can even leave off 2 and 3 and just press 4,up+4 in the begining and still do the same amount of damage if not more. If you know the timing well enough you can poke them with 1or B+1(if you really get it down) just after the pop-up then go into the chain and make the damage in the deep 44-48%
I noticed about popups online, popups go a little higher. I went in training and versus to check, then online and it seems they go just slightly further up.

No, I mean the pop-up attack combos u learn in training.

Yes I do uses the popup combos I learned in training, I love then to death, especially Mileena's and Tanya's, that the way I finally beat Shang Tsung in the Netherealm with Shujinko.

Pop ups are great for starting combos and using up your opponents breakers. I use kabal, and in every stance i use pop ups to start my 30% + combos. Liu kang also has his famous(and annoying) pop up in his first stance, so you can only how great pop ups are. the first move used to pop people up with arent that weak either, and some can hit your opponent when he/she is ducking too. I love pop ups... (and i mean MK FIGHTING pop ups when i say that)