A recent Dragon had a great article on variants to the Summoning Mechanic. Our group uses these rules exclusively now. The new rules base Summon Monster spells exclusively on CR.
Elementals (since they have tons of HD and DR to boot) are considered two CR higher than comparable monsters.
The chart, IIRC, goes something like this:
SM IX CR 12-13
SM VIII CR 10-11
SM VII CR 8-9
SM VI CR 6-7
SM V CR 5
SM IV CR 4
SM III CR 3
SM II CR 2
SM I CR 1
Astral Devas have been removed (CR 14), but otherwise monsters have been bumped up or down -- Rakshasas, for instance are now on SMVII (CR 9).
The article also talks about slapping on templates including celestial, fiendish and all the ones in the MotP. Also, it has variant rules in case the DM or players think the ability to select so many templates is too powerful.
Finally, there are guidelines for Summon Nature's Ally as well.
All in all, a great article and worth picking up.
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To answer your question, I think that the original Summon Monster series of spells is quite underpowered.