Eh, I think they are slightly underpowered.
In my opinion, they are not supposed to be very good as combat spells, and for this reason I think the current power level of the summoned creatures is fine.
The immense versatility of the summon monster spells more than makes up for their weakness in direct combat. It's worth repeating once again: they are not as powerful as a fireball or meteor shower, and they are not supposed to be so. You can use summons in so many wildly different situations that demanding for them to be good in combat too would really be too much.
However, the duration is far too short for a spell that should be good outside combat, and it's practically ridiculous at low levels, where you don't even have time to explain to the creature what it's supposed to do. I would bump the duration, maybe to concentration + 1 round/level.
Also, they suffer from the fixed list. It means that buying new monster books doesn't add to the creatures you can summon, and it means that all summoned creatures will always have no class levels and minimum HD. Removing the fixed list in favor of a CR-based system would make them much cooler and slightly more powerful.