Basically, it's a very strong Crossbow weapon from Diablo II.
Type: Ballista, Crossbow class
Item quality: Exceptional
Best Drop Probability: Diablo (Ni) 0.043719% , Baal (Ni) 0.042333% , Mephisto (Ni) 0.042%
Two-Hand Damage: (85-102) To (142.5-415.5) (113.75-258.75 Avg)
Required Level: 41 , Required Strength: 110 , Required Dexterity: 80
Base Weapon Speed: [10] , Firing Rate: Very Fast: All
+150-200% Enhanced Damage(varies)
+ (2.5 Per Character Level) 2.5-247.5 To Maximum Damage (Based On Character Level)
Adds 32-196 Cold Damage - 4 Second Duration
Piercing Attack (100)
Freezes Target +3
+75-150 Defense(varies)
+35 to Dexterity
80% Increased Attack Speed
This is probably one of the most debated weapons in the game. It has spawned a character sub-class based purely on this weapon (the Burizon), started countless flaming wars on most forums and to some degree even shifted the balance of the game. The reason? It is all too common because it is an exceptional unique, so you can even find it on nightmare difficulty where many monsters are able to drop a ballista class weapon. This weapon should definitely be classified as elite making it almost as hard to find as Windforce and Eaglehorn.
Now lets look at the long list of it's merits. First of is the insane physical damage delivered with an increased attack speed of 80%, which is very high for a crossbow class (even tough most bows still fire even faster). Moving along the list of mods we encounter the adds 32-196 cold damage, which makes it usable in itself even against physical immune monsters (to some lesser degree anyway). To complement this cold damage is the freezes target +3 mod and the healthy boost of +35 to dexterity, adding to the already high physical damage.
The icing on the cake and this weapons probably best mod is the piercing attack (100), which makes bolts fired from this weapon pierce every target unerringly. Try this on a pack of monsters to see the effect if you do not understand this concept.
When considering the socketing options (and other equipment) there is another blessing tied to this magnificent beast. The mod of 80% increased attack speed makes it very hard to make it even faster, because of the diminishing formula on IAS thus freeing the socket option for use with an enhanced damage jewel or a rune, although some still advocate the faster the better approach (and rightly so).
The rune option might quite possibly be the best one, with many runes having great mods for a ranged type of weapon. Adding a Nef is the most obvious choice in order to gain the Knockback ability, always a desirable trait for ranged weapons, unless you plan on using a piece of equipment that already has that mod. Some of the really rare runes such as Ber (20% Chance of Crushing Blow) or Jah (Ignore Target's Defense) or even Lo (20% Deadly Strike) might also seem as good choices, but have in mind that these are VERY rare, and maybe better used for trading or for possible future rune words. On top of that chances of Deadly Strike and Crushing Blow are penalized when used in conjunctionwith ranged weapons.
A very viable solution might be to put a source of mana leech into it, like a perfect skull (or Vex rune), thus making it possible to equip your character with items that do not have this must-have-ability, in order to maximize your potential in, say, magic find.