I would definitely play 99 from any position (or even 88 from UTG). The hand itself is certainly not a strong one if it remains unimproved and pre-flop it might be near the bottom of my range (depending on table dynamics) but I don't think it's ever unplayable.
Like all hands I will play, if I'm first to open (I'm either UTG or it's been folded to me), I'm opening for whatever the standard table raise is. If I get re-raised then calling vs. folding will be based on effective stack size between me and the raiser: I won't play without taking the lead (being the last raiser) unless the bet size is less than 8% of the effective stack: if there isn't enough money available to get paid off, it's not worth playing. I'm playing 99 with the intention of hitting a set and getting all-in. If I don't hit the set (which is the case 88% of the time), how I play depends not on my cards but the situation (and is no different than how I play AK that didn't hit).
Based on another posted answer there seems to be a question of ability to get paid off here so I would like to address my thoughts on those points.
99 against 7 random hands is 19% or about 4:1.
This hand, in my mind, is being played for set value. If we hit our set (on the flop), we're now approximately 68% to win the hand against 7 random hands. If we reduce the range of those 7 other players to reasonable calling ranges our equity does not drop significantly. That said, I don't know why 7 was chosen; if I'm in a game where I can expect 7 players to see most flops, I'm not going to stick around. It's worth noting though that if we end folding most players, when heads-up we're a slight favor to win the hand without improving it.
There is a saying
You win small pots with marginal hands and lose big pots
And in this case it is so true.
Since I'm playing for set value, I'm not concerned about a marginal hand at showdown. Of course, certain board dynamics can change that significantly, such as an open ended straight draw or 4 to a flush. Fortunately these scenarios do not appear often enough to be concerned with them pre-flop.
Position, position, position
Even if you hit a set you lose 1 bet by being out of position
We can certainly miss out on one bet when we place it and our opponent(s) fold, but to think we lose a bet due to position implies our opponents are going to take the lead when we're in position and we really cannot count on that. Beyond that though, position is always an important factor and we can never do as well out of it than in it. This is not, in my opinion, justification to fold 99 pre.
Most of the hands that are a coin flip or will dominate you will be in the pot.
Any bigger pair will be in the pot (you are dominated and out of position)
Any hand with two cards over 10 or over will (likely) be in the pot
Of course we can be in the hand with people holding higher pairs, but we don't expect to win every hand we think we're favored to win. Being 68% to win when we hit our set is pretty powerful, and to alter our play for the 32% of time we are going to lose anyway is too tight; it leaves money on the table.
You will get some K little suited and A little suited (only 2:1 against them and they the are not going to stay around unless they hit A, K, or a flush draw)
If we hit our set, we're looking to get stacks in the pot. If they hit their flush we're going to lose our stack; this is the risk we're willing to take (and by only committing 8% of the effective stack pre, we leave room to make up for the times it happens).
Most of the hands you dominate will not be in the pot
I get this is maybe circular but I would only play 99-55 and low suited connectors in mid to late position.
That's ok if they decide to fold, but it's certainly not a guarantee. If we're UTG and UTG+1 has such a hand, they'll probably fold. Any player with a pair that folds to a single raise and two callers is ideal to have at the table because they'll fold any time they don't have the nuts. These players are great sources of money but unfortunately they're extremely rare.
If the board flops dry you are not likely to get any action.
We want action of course, but if everyone folds that's ok too (even if less ideal). Reinforced again: we're playing for stacks. We can't get stacks into the pot every time (and even when we do, we don't win every time), so we're not stopping once we've hit 8:1 on our initial raise (and if we're fortunate enough to have many players in, we're close to our 8:1 before the flop even gets seen). That said though, even on a dry board we're going to get callers because by coming in with 99 for a raise (and by raising every time we open the action... never limping in) because they're never going to put us on a set with this hand. Most people that called our pre flop bet have at least one (and often two) overcards and they think we're just c-betting without anything to back it up -- something we'll do in certain situations but not as often as they think.
If the board flops wet yes you are going to get action but you are also likely beat.
If we have more than two opponents and we didn't hit either our set or at least a hand with 8 or more outs, we're not going to c-bet here. If we did hit one of these favorable situations, we're going to bet and getting action doesn't by any stretch mean we're beat with certainty; we're only 32% to lose by showdown (and that often requires turn and possibly river cards to get to that point). We'll get called with several draws because even though we priced it such that they don't have odds to get there, our opponents will call anyway (and if they don't, that's good too because it gives us an instant win).
If you hit your 9 on the flop
Again not going to get paid off on a dry board
And you are going to lose big to bigger set, straight, or flush
The only way you get paid off is a lower set and two pair and even then you are not going to likely to get paid off big
Yes you get paid off by bigger pair but it not likely be a big enough payoff to justify the 8:1 to get set on the flop.
I believe these have already been covered but to reiterate: It's absolutely true that we are not going to win just because we hit our set, but we're going to win by a healthy margin when we do. When we're up against a straight or flush it's almost never on the flop but rather the turn or river that got them there, and we've had time to build a pot. The very interesting thing about the river when we've got a set is that we have more outs to make a full house (or quads) than our opponents have to make a flush or straight. If our opponents are drawing for a flush they often have fewer than the 9 outs they think they have. When the stars are lined up just right, we hold one of their outs and two others will pair the board for our full house.
The bottom line is that 99 is not a great hand from any position and, like all hands, especially from early position. However, if played properly it is a profitable hand. If the hand is folded when in early position this profit is lost but it's by far the greatest mistake one could make with the hand. Every hand in poker needs a plan to be played properly; this one from UTG probably more than others. Improving ability to play 99 early should not be priority when weighed against many other concepts, but it should not be tossed out never to become part of one's arsenal either.