My biggest concern about guide services is with regards to their ratio of guides to clients and the risk of turning a healthy rope team around because 1 person has a headache. I've seen this first hand and have also heard numerous stories from friends.
One of the guys on my Denali trip attempted it previously with a guide service. They were at high camp (17.2K) waiting out several days of poor weather. When the weather finally broke, the guides informed the entire group that they were out of time and had to go down. Nobody was even sick in that instance. They still had plenty of food/fuel and they were forced to head down because of the guide's schedule. There's was nearly a mutiny.
I would definitely ask how they make those decisions before you hire a guide service. Is there a set time to summit? Are there any extra guides to help sick members or do they turn an entire rope team back when 1 person (who you don't even know) gets sick.
Obviously, if someone is seriously ill their safety comes before a summit bid. I'm not suggesting otherwise. I'm talking about someone with mild symptoms like headache or nausea that can be walked down by a spare guide. When you pick your own teammates and climb unguided, you accept the risk that you have to help each other if anyone becomes sick. You have a strong incentive to chose capable partners. However, when you pay $7K for a guided trip and you're climbing with a bunch of strangers, you hope the guides can shoulder the burden for those things so somebody who perhaps doesn't even belong on the mountain doesn't ruin your chance to summit.
