Was it rare for defenders to repel boarders effectively? Did tactics exist other than a melee for attack/defence during boarding?
Is there any evidence of a ship on the losing side of a gunnery battle managing to achieve victory through boarding?
Boarding was very common partially because the crew of the victorious ship would be allowed to keep profits gained from taking an enemy ship. So there was an obvious advantage to not battering an enemy merchantman more than you had to. Boarding could be a very bloody pursuit, but theres far less damage done to the ship and cargo.
Also, not only must you capture a ship, but you must also get it back to a friendly port. The more you batter a ship the harder this will be.
Plus, gunpowder is expensive and cannons are very large, heavy, and difficult to use. Navies would only provide Captains with a small amount of powder to use for practice and training their crews. A five hundred pound cannon violently shooting back across a heaving and rolling deck is very likely to hurt one of the people working the gun if they're not used to it. Drafting sailors also meant that many navy crews weren't really experienced fighters, even if they were sailors by trade (and many weren't).
Although Captains who were well off financially could buy powder at their own expense and use it for training, and many did.
Probably the most famous act of boarding in the face of overwhelming odds is when the British Speedy, of 14 guns, under Thomas Cochrane ran afoul the Spanish frigate El Gamo of 32 guns.
To put this into further perspective, Speedy fired a broadside weight of metal of about 27 pounds, while El Gamo fired 190 pounds. Cochrane's crew also only amounted to 54 men, while El Gamo held 319.
Realizing he was pretty well boned, Cochrane boarded El Gamo. In the opening few minutes of the battle the Spanish captain was killed, adding to the confusion and someone tore the colors down off of El Gamo's mast making most the crew think that the ship had surrendered.
This is probably the most famous case of a successful boarding action, but of course its the exception rather than the rule.
There was also The Battle of Cape St. Vincent, when after an hour of exchanging fire, Horatio Nelson boarded San Nicolas (Spanish, 80 guns) from his battered Captain (British, 74 guns) and then, after taking San Nicolas, continued on to board San Josef (Spanish, 112 guns) and taking that too.
As you can see he basically captured a ship and used it as a bridge to capture another ship.
But once again, these stories go on to be famous because they are exceptions to the norm.
Boarding was in reality of course very risky for a lot of obvious reasons, for example it would be very bad to have most of your hands lined up to board the enemy ship, only to have the enemy ship open fire with a broadside of grapeshot (hundreds, or even thousands of little iron balls depending on the size of the gun, which are loaded in lieu of a traditional cannon ball).
Regardless though, boarding was very common. Although its also important to mention that if a ship has less guns, than its also got less men (although privateers would often hold much larger crews than navy ships since they sailed purely for profit and were even less reluctant to spend money on gunpowder and to batter their prizes than navy ships).
This means that boarding wasn't really a viable alternative to ships that were out gunned.
Links to the above mentioned actions:
http://en.wikipedia.org/wiki/Battle_of_Cape_St_Vincent_%281797%29#3:00_p.m.