In the game I run, we have a Druid Summoner - she's been very effective. The larger animal summons are quite dangerous and some of the special ability monsters are nasty - elementals and phase spiders have both been put to very good use in appropriate situations.
I'm finding them rather useful at the moment. I'm playing a 13th level
Alienist. The augmented pseudonatural summons are just mean. True strike is a strong ability for the summoned creatures to have. However, this does diverge significantly from the regular summon spells.
IMO, the Augment summoning feat makes a marked difference to the spells. At lower levels the +4 strength makes them much more effective in combat. At higher levels, the +4 con can make a real difference to the durability. Then again, a 2 feat investment ought to be good.
I think the best value for spell summons are ones that can grapple, some of the bigger creatures are easily capable of locking down human sized opponents. Whether a GM will allow some of the more stupid creatures to grapple is another issue all together.... My favorite is most likely Fiendish Giant Constrictor snakes from the SM VI list.
The full round casting time can be a bit of a problem in all cases, I find getting out of line of sight of oppenents to be vital, before you start casting?
Agree with the other posters. If a player has not worked out the stats for a summoned creature they normally can't use it for combat.