I don't think that summoning is underpowered per se- it's just that people don't know how to use them as effectively, and/or dismiss the utility of the summoned creatures.
Firstly, I think that many of the creatures summonable, certainly from about the 3rd level list, can indeed hold their own against many opponents. That's not to say that they are of equivalent power- that would be ludicrous. If a CR5 conjurer can summon a CR5 creature with one of his spells, and he has three such spells, then there is clearly something going wrong. However, nearly every summon creature list (except perhaps I) has something useful. And remember- Augment Summoning is the conjurer's best friend.
Lemure: So they're not going to end the world, but Damage Reduction at level 3 is certainly going to throw a lot of opponents.
Fiendish Wolf/Hyena: The Trip ability is quite useful.
Celestial Black Bear: With three attacks, these do a respectable amount of damage, particularly if summoned adjacent to the enemy wizard.
Celestial Bison: Not powerhouses in terms of damage, but DR at level 5 is often still hard to deal with, and their resistances and special abilities are also nice.
Fiendish Dire Ape: DR, Smiting and some powerful attacks
Fiendish Dire Wolf: Ditto. Trip makes them particularly nasty against back-line types.
Celestial Dire Lion; 5/+2 DR is going to annoy many opponents at this level, and they can suck up a decent amount of elemental damage. Attack power is also decent.
Fiendish Girallon: Summon adjacent to opposing wizard and watch the chaos ensue.
Celestial Dire Bear: 10/+3 DR, SR 24, significant offensive power and huge number of hit points.
Chaos Beast: Can really put the wind up people with poor Fort saves.
Huge Elemental: More hit points and HD than the opposing fighter and very great offensive powers (three attacks with potentially 4d8+6 damage)
Fiendish Dire Tiger: SR 25, DR, elemental resistance, decent offense.
Greater Elemental: Phenomenal offensive power and vast number of hit points.
Elder Elemental: Likewise.
However, the best bet is when used in conjunction with the 'flooding tactic'. A hasted conjurer can summon 1.5 monsters/round, so within two rounds of combat, he can potentially have three of his top monsters in action (assuming he's a specialist with a decent Int). The point is- he keeps going. Keep summoning creatures unless you have overwhelming force. This is also great with Wall of Force. Have a huge Wall of Force, summon all your creatures, then drop the Wall and let all hell break loose. Having half a dozen Chaos Beasts suddenly on you can cause all sort of trouble, and half a dozen Elder Elementals is going to cause even a 20th level party to think twice.
Spell usage has been done to the death, so I'll add my 'WHAT HE SAID' banner. Higher-level celestials, devils and demons excel at this sort of thing: erinyes can animate dead at will, which saves time and effort for the necromancer and the vrock's mass charm at will (8th level spell) is not to be sneezed at.
Finally, there is the question of general utility. If you need a door bashed in at low-levels, you could do worse than a celestial bison. Need a trap triggered? Blow a Summon Monster I and send it a celestial badger. Not sure what's round the corner and don't want to find out- use a celestial eagle. The uses for summoning are as many and as varied as the summoner's imagination.