Ah, constitution. Constitution has no other use except one. The holy grail of it all; hp gain. More hps is loved. By everyone. The guideline is simple, the more con you have, the better chances you have to hit the upper scale of the hp gains.

Although, HP gains (as well as mana gains, really) are something of an anomaly. They're very fickle at times, and there's no true consistent pattern in them. Even with maxed out int, wis, and con, I have still managed to get some terrible gains at times. So don't lose heart if this happens to you. All the gain stats simply give you a better average of totals in the end.

With that thought, it's certainly up to you what to do with con. I, personally, always train con because I can't imagine myself being some sickly Raistlin running around Avendar. Extra hitpoints are always good, and I've definitely noticed a difference between nefortu and shuddeni when comparing HPs. But in relation to all of the classes, con stands as an equal importance to everyone.

You can't do much of anything once you hit zero hitpoints, after all.

Charisma

Charisma is just weird. Simply weird. Only one class should really obsess over charisma, and that's bards, for obvious reasons. Other than that, charisma is another whimsical judgment call (starting to see a trend here?).

Charisma has a direct relation to your haggle rates. Better charisma means better haggling. If you don't have the haggle skill, then this doesn't really concern you. If you want some cheaper prices, then go ahead and raise charisma. I probably wouldn't. Just take the extra time to gather a few extra coins and you've already covered the difference anyway. Note that void scholars can't haggle their way for cheaper spell components at the guilds, so don't base your decision off that either.

Charisma also (allegedly) has an effect on mob hostility. If a group of four of you walks into the room with an aggro mob, the aggro mob is supposed to lean its initial attacks to those with the lower charisma. To be honest however, I haven't seen this happen very often, and even if it did, the tank (who should have rescue) should be taking the aggro to himself. So, this is pretty useless as well.

Now, there's a definite use for charisma if you've chosen good (aka lightwalker) as your alignment. The PROPORTION of your charisma to racial max charisma, actually has a hand in deciding how high of a level mob you can request form. Request is a fickle process in itself, as the max base mob-level you can request from is based off your own current level. From there, it is modified by charisma. Note that I said proportion though. It's not that higher charisma races will benefit more than lower charisma races in this case, it's that the closer you are to your racial max, the better your chances are. So, if you have max charisma with a ch'taren, and max charisma with a kankoran, both have the same level ranges for requesting items.

Thus, in the end, charisma is incredibly useful to bards (just look at all the helpfiles on bard skills, and see the primary attribute, also charisma apparently has an effect on music strengths). It also has a use to lightwalkers, but I would heavily suggest swapping charisma gear for when you want to go requesting. Because of the general weakness of the charisma stat, it's not uncommon to find a +2 or +3 charisma item laying around.

One final note about charisma though, one should never write a description of a beautiful character if you have a horrible charisma. Like intelligence, this will eventually be noticed, and it will be frowned upon. Fair warning.