At what level does it become worth it to begin investing in Summon Monster?
In other words, which Summon Monster spell allows the summoning of monsters that will actually be able to contribute at the time when you are 1st able to get said spell?
Playing a Sorcerer, if that matters. Thanks! ^_^
How creative do you want to be?
For me, level 1. Summoning a poisonous viper amounts to a shot at poisoning someone from some ways away. You can even do it while vanished, though that won't matter much until at least 2nd level. It'll only work on someone with poor armor or poorer luck, as it requires a crappy attack roll and some failed DC 12 save (with Augmented Summoning). At the least, the guard your pet bit might leave his post to deal with the possibly venomous wound. But it may be worthwhile if you stay hidden and the enemy is flatfooted.
Out of combat, summoning an eagle has decent utility for a 1st-level party, even if it only lasts 6 seconds--if you can communicate with it, which is a pretty big "if".
Summoned creatures can also make for more convincing distractions than, e.g., ghost sound--again, if you want to stay hidden.
If you're looking for raw combat prowess, you might as well wait until you can get a lantern archon or something. Stick to magic missile.
Once you're second level, summoned creatures get to make more than one attack roll, which makes them better than most first level attack spells.
At the other end, there comes a certain point where summoned monster are less and less powerful than the party and the foes you face unless you've invested in Augment Summoning and such things.
Even then, they're useful for battlefield control, especially on non-flying enemies. Summon a horde and provide flanking buddies for your entire party.
Don't forget the plethora of SLAs, Ex and Su abilities. It's a swiss army spell that is occasionally the best choice, but only very rarely the wrong one.
I agree with Ross, but I usually wait until level 3 before I start memorizing them.
(Also note that you can summon monsters with templates. The smite ability ignores DR, which makes the monster really effective against enemies with that type of defense.)
