The fact that elementals are immune to critical hits, stunning, and such is usually a fairly moot point for the lower level ones. Most of the small elementals have around 10 hp so that critical hit gets stopped, but at 5th-6th lv it is most likely going to die from one hit. This factor really makes the Summon 3, 5, and 6 elementals utility creatures rather than combat monsters.
Unfortunately the short duration of the summoning spells means they can't run any long errands for you. I wonder why no one has published a feat to make your summons last longer. Psions have a feat from the WotC site that extends the duration of their Astral Construct powers from rounds to minutes. I wonder why they haven't bothered to do a version for arcane and divine magic.
When I played a druid I got much more use out of the Thoqqua and the Ape as my summons of choice for a 3rd lv spell. The thoqqua is invaluable as a digging tool since it's burrowing leaves a 1' wide tunnel. If the distance you want to go is fairly short (and you speak ingan) you can have it burrow back and forth through the rock to form a tunnel that people can fit through.