I had a few initial blips but for high traffic, server-intensive sites I have found them excellent & they've removed a lot of stress for me.
Previously I'd a 16GB dedicated server (managed) for ONE site + a good reseller shared account for some smaller sites - this was costing me about £400 per month & the 16GB beast was falling over during spikes....loads of stress & £££.
My wpengine costs $249 and is stress-free with loads of extras (such as backups, restore points & staging areas).
Highly recommended for anyone in my situation; half the price & 1% of the stress so far (about 6mths).
so long as it's not over 25,000 visits amirite?