That's 99.90%, 99.94%, and 99.88% (for the month so far), or simply 99.91% for the entire period.
So, what would you consider "rock-solid"? Personally, I'll echo what was said in the other thread - 99.91% is much better than what I could accomplish on my own, so I'll continue to trust my business to them.
Regardless, 99.9% is really not special or even acceptable from a high dollar host like Heroku. Cheap shared hosts like HostGator can give you 99.9%.
I'm pulling for them, but they've got some work to do.
However, Heroku appears to have no SLA, so its a moot point anyway.
I may be showing 48^ errors in my per-minute checks but those errors occurred in at least^ nine separate blocks across an eleven hour window.
In other words, we are probably looking at around ten hours of issues, even if not outright outage.
^ I disabled the checks at times because it was just a waste, so the true stats are likely worse.
99.5% (or less) = unacceptable 99.9% = average 99.99% = exceptional
0.5% downtime = 216 mins/month (almost an hour a week!) 0.1% downtime = 43.2 mins/month 0.01% = 4.32 mins/month
Getting to that last level is certainly very hard, I hope Heroku will get there, but the service is relatively new, fairly cutting edge, under active development, etc. I would not entirely expect them to reach "four nines", but I hope it is their goal.