The net rules seem a little fuzzy to me.
Your foe is entangled so he can only attack the net at -2. Objects just lying around have an AC of 3 +/- size modifier (a large net would have an AC of 2), but the net has a rope that the wielder holds onto. This means by the rules the foe must attempt a Sunder maneuver, his CMB vs. your CMD (which provokes AoO) if he wants to attack the net to break free.
It requires a Sunder combat maneuver specifically because the "Breaking Object" rules say you need to use that maneuver to damage any "worn or held" item, holding the rope technically counts. I could easily see a DM ruling that the foe is attacking the net, not "sundering" the rope, but that is not what the rules stipulate.
Also in the rulebook there is a bit about ineffective weapons that specifically points out that bludgeoning weapons cannot damage ropes. That rule is wide open to DM interpretation because it doesn't list every possible ineffective weapon situation. I would suggest that piercing weapons can't effectively damage a net. Historically, in the Bronze Age, nets were very effective since almost every soldier/gladiator/warrior carried bludgeoning weapons or spears or thrusting swords (basically no sharp edge) - almost nobody had any kind of cutting edge so getting out of a net was next to impossible.
So (arguably), damaging a net should require some kind of slashing attack (or magic like fire or acid or force), you must do 5 HP damage as a Sunder maneuver at -2 to to the CMB (unless the attacker doesn't hold onto the rope in which case it's a normal attack at -2 against AC 2).
Your opponent could also try Escape Artist (DC 20 isn't too hard for anyone nimble enough, maybe easier than fighting your way out with a mace) or could try to burst it with a STR check but would need at least a 20 STR to have a chance, and even that requires a natural 20 on the roll.
Before I make a net-using character, I would clear this up with the DM in advance. He might see things very differently and allow almost any weapon (mace, fist, head butt, whatever) to attack the AC 2 net without requiring a Sunder maneuver, in which case, all you have is a one-time use item that makes an enemy waste an attack breaking free (but you waste an attack entangling him in the first place).
First of all, the rules that DM_Blake is referencing are found here, PFRPG Core Rulebook: Additional Rules.
Cutting the net itself is indeed a Sunder combat maneuver vs the object's AC. As DM_Blake said, a net's AC is 3 + size modifier, but that is only if the net itself is stationary.
I believe that the net's size, however, is variable depending on the creature it is entangling. Laid/stretched out, yes, a net's size is the size of the largest creature it can entangle (so a net sized for a Medium creature would have a maximum size of Large, as Large is the largest creature it can entangle), but while entangling a creature, I would say that the net's size would be the same as the entangled creature, no?
I do not believe, though, that you have to sunder the net's rope that the opponent is holding in order to escape the net. Sundering the rope would only prevent the opponent from keeping you on a leash, not allow you to escape the net. In the case of sundering the held rope, yes, that would be against the opponent's CMD, not the net's/rope's AC.
Also, it says in the rules for Smashing an Object (emphasis mine):
Smashing a weapon or shield with a slashing or bludgeoning weapon is accomplished with the sunder combat maneuver (see Combat). Smashing an object is like sundering a weapon or shield, except that your combat maneuver check is opposed by the object's AC. Generally, you can smash an object only with a bludgeoning or slashing weapon.
And since you can't cut a rope with a bludgeoning weapon, you can only cut your way out with a slashing weapon. Personally, I'd allow a piercing weapon too, but the piercing weapon only deals half damage each time.
So, in summary, destroying a net is always a sunder special attack (not a normal attack vs AC), made against CMD = 3 + net's size modifier, and should be done with a slashing weapon.
I think we're all agreed on the need for the net to be sundered. However, if you could break free from a net (held by an opponent) without having to overcome the opponent's CMD and just the net's CMD, then my general opinion of nets just declined even further.