Force 9 spirits are, of course, rather ridiculously powerful. An air spirit has a ranged attack of 18P/-9 (Elemental Attack, electricity) with 21 dice and a melee attack of 27S/special (Engulf + Energy Aura). It also has initiative of 22 + 2d6 (before boosting with Increased Reflexes), 18 hardened armor against most attacks, and a defense pool (Rea + Int) of 22. Oh, and it makes your whole team invisible with Concealment (-9 dice to enemy perception rolls). Other spirit types are comparable.
Here's a sample character built for summoning them (among other things):
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Increased Willpower can be sustained with Focused Concentration, giving him a Willpower of 9 (maxxed out at natural +4). That plus his natural Charisma of 8 is a drain pool of 17. Summoning a Force 9 air spirit with 19 dice and then resisting drain with 17 dice results in an average of 3.3 services and 1.5 stun damage from drain (53% chance of no drain at all). If he's about to take drain damage, he can spend Edge (of which he has plenty) on Second Chance -- that reduces things to an average of 0.3 stun damage (87% chance of no drain at all). With some extra time, he can also quickly rest it off (adding Increased Body via a sustaining focus plus reagents gets him to 14 dice for healing stun damage).
Binding a Force 9 spirit is scary stuff, but just summoning a new one whenever you run out of services is very doable and incredibly powerful.
A couple other notes about the character: While he's incredibly frail physically, he's actually very hard to hit unless he's had to shut down his sustaining foci. If he's expecting combat, he'll generally be sustaining Increased Willpower (via Focused Concentration) and then Combat Sense on himself and Increased Reflexes on both himself and his summoned spirit (using sustaining foci). Those last three would be cast at Force 1 so they can fit in the sustaining foci but with reagents to increase their limits to 9 -- reagents are basically ammo for spellcasters. That gives him plenty of initiative to use Full Defense; that plus Combat Sense that typically gives him 21 dice for avoiding attacks, plus lots of Edge if things look ugly.
He doesn't have any weapons or combat spells. He'd be perfectly competent with combat spells, but the spirit will still hit much harder and Control Thoughts (with a decent Initiative) actually seems like a more useful spell in a fight -- I haven't tried that in actual play, though.
Edit: Since the sample character above seems particularly contentious, here's another one that's more well-rounded. He still won't do well at arm wrestling, but he's only a bit below average physically, and he adds some interesting capabilities: He's a great scout (strong detection spells plus perception to go with spirit Concealment) and all-around healer (17 dice for First Aid with a medkit, plus Heal and Resist Pain). He can still summon Force 9 spirits -- only 2.7 average services with no Mentor Spirit, but he's the same on drain resistance. Instead of a natural 8 charisma, he boosts both of his drain attributes via spells for the same total of 17 dice. He's tougher in combat with the same suite of defensive spells (Combat Sense, Increase Reflexes, Increase Willpower) plus better Body and Int + Rea.
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