Here is how Spirit Pact works:

First, to use it, you have to have a summon out. The summoner does not need to be the one with Spirit Pact (unless you're actually playing the 3DS version, which may have changed some of the other details), but it has to be a summon from a spell. (NPCs and Replicas don't count.) Furthermore, using this skill will cost half the user's SP and cause the summoner to lose the summon spell (so if you want to summon another, you have to find another monster to contract). The monster then becomes a regular party member, filling the first empty slot (or going to the tavern if there is none).

The monster keeps its name and appearance. If you go to change the appearance, the only options (unless you've found a certain postgame treasure) are the class icon and the "previous life appearance" (or something like that) which gives you the monster's portrait.

The character's class depends on the monster in question. It can usually be predicted by looking at the monster's skills, and seems to default to fighter. (This turns out to be spectacularly wrong for the Werewolf Archer, to the point where it's probably a mistake.)

The following traits are kept:
Alignment
Gender (- becomes ?)
Breath type (Fire if none) (Note that non-Dragonewts still can't breathe.)
Status Recovery
Turn Recovery (only way to get 3 digit turn recovery)
Magic Resistance (except Summon, which becomes 100%)
Attribute Attack/Defense
Status Attack/Defense (this includes Beheading)
Known spells (can learn more spells of the same level)
Weapon Break trait

The following are not kept:
Race (usually becomes Human or Devilish depending on the monster)
Class (depends on monster, as stated above)
Level (becomes 1)
Stats (becomes racial base stats)
HP (becomes 8)
Number of actions
Skills (becomes those of the class, plus a random EX skill)
Enemy only abilities (i.e. Draw)
Equipment
Item Bag Contents (if a Servant)

Also, changing a spirit pact character's class will cause the character to lose most if the kept traits. (Spells and I believe breath type are kept.)

As a side note, I think some enemies have more summon resistance than they should, including all the Cleric monsters (that I have seen) and most of the Mage monsters (I consider Don Mags's 80% resistance to be justified, given how powerful it is after Spirit Pact).

Dragons tend to become Dragonewts. Some of the Mage and Cleric monsters become Elves. There are a few Faeries as well (Pixie not being one of them). There are no Dwarves, Gnomes, or Hotlets. I don't think there are any Werebeasts (otherwise Werefolf Archers would become them, but then again they messed up on that monster's class).

There are no Goblin, Ogre or Magic Doll monsters, sadly. Those races are only available by hacking, and I believe their base stats are the same as in Elminage Original.

The Brawler trick only applies to using Contract, not to Spirit Pact. The method is to (using fixed turn order) have the brawler attack and immediately after words, use Contract. If Clean Hit triggers (which isn't as likely as one would like), the enemy's Summon resistance will be lower for the next action (in this case, the Contract). I have not actually used this method, but from testing with the Dice of Fate, it looks like you could have the brawler use the Aura Punch to avoid killing the enemy. (What a strange weapon! Quite nice, except for it being totally ineffective against everything.) I only consider this worthwhile if the enemy's summon resistance is 90% or higher. (Having your Summoner be at least 10 levels higher than the enemy is more important.)

The only way to see your SP outside of battle (to my knowledge) is to pretend to use an SP using skill that can be used outside of battle (like Spirit Pact or Replica). Maximum SP seems to be equal to the square root of the character's level, but no higher than 99 (but are you REALLY going to take the time to reach level 10,000?).

One more thing I should probably mention: The special traits of spirit pact monsters do not stack with equipment. You can see that on my Dragon Lord with 28 turn recovery. Zer Holy Saber's +20 turn recovery does not stack with zer natural turn recovery. Also, they don't stack with Special Resist Up or Court Sanctuary, but do with Improved Beheading Chance.

As for the number of pages/replies to the thread, that seems to be a forum bug,

One update: The Weapon Break trait is retained. I now ave a Nue Samurai who is able to break enemies' weapons with a normal attack.

*edited*