Currently I'm the sole DBA on my team and quite frankly, I'm not a fan of the situation. I don't think my bosses are either, but budget and contract things are keeping them from getting me even a part-time backup. So far, I've not taken any sick days (haven't gotten sick,) I've only missed one day when my car wouldn't start (dead battery,) and I've not taken any "mental health days" (not that I've not been tempted, but it's kind of a bad idea to call in for the day when you've already been at work for a couple hours,) so I'd say I've been quite reliable myself. I'm working on building up some better documentation of the systems, both for my own notes and as a "I got hit by a bus and won't be able to work for several months, someone else needs to handle the servers" sort of situation.
I do have the advantage of only being required to be available during our normal business hours, but I've made it clear to them that if there's a server-stopping issue (and I've had one of those) in production, I'll work the problem regardless of when it hits. Unless I'm somewhere that I can't work remotely (vacation, out of the country,) then it has to wait until I do get somewhere I can work the problem. As for performance my understanding from the bosses is that the customers are quite happy with the performance of the systems, and the availability has (knock on wood) been very high.
I think the biggest challenge to me being the sole DBA is going to be this year, though, in August, when I'm taking a 2 week vacation and travelling down south. I don't plan to take my work laptop along, although I may take my work phone along, it'll spend 99% of the trip in the room safe only occasionally getting pulled out for me to check e-mail. I'll do my level best in the months between now and then to get everything in order so that best case when I get back I'll just have to catch up on a stack of e-mails, but there's always going to be the unexpected...