Also, altars come in different qualities too. You get two altars from completing the deity quests, the second one is better: meaning it requires less faction to buy the "miracle spells".
The handcrafted version is better though, but most likely quite expensive.
The miracles you get at the highest level are quite good, and after the third (or was it second?) quest, you get a pet that will boost stats. ie: not useless like most pets are.
Evil or good characters are not able to join every deity, you might have to betray to get the deity you like. Also, you can switch deities and still keep the cloak you got from another deity. Heck, in theory you can gather all the cloaks. (those are actually very good items to have, definitely worth getting the group for it)