If you log uptime with a 24x7 type of service, I will be surprised if you see 99% for a month.
Response time is critical if WP (or any other db-driven app) is a key work environment for 1+ users, if you want visitors to interact with each other (or you), if it's an ecommerce site, etc.
If you have lightweight pages, if you don't use a lot of plugins, if you use caching well, and if you are not doing much more than publishing posts now and then, the grid is workable--albeit more costly than faster and cheaper hosts.
If you have much going on in WP, an SQL container on the grid will make it a lot faster (and double the cost) but still not fast to the point that you consistently perceive an instantaneous response in the range of half a second. If you compare with a really fast account, you will start to notice the difference.
Try the latency plugin on vanilla installs on different hosts. I did that along with http response logging with 2 shared hosts, 3 different (gs) accounts, and a moderately trafficked WP site in production on a (dv) earlier this year. The grid's sql latency stuck out very clearly. You can see this just by hitting WP out of the box with 500-1000 page requests from a load testing script or webwait.com.
The sluggishness of the grid stands out strongly in a way you can feel when you are spending hours working in the UI. Especially with heavier applications like a customized Joomla or Drupal site, on the grid any backend page load usually takes at least 1s+. I'm working with a new install of Joomla+Virtuemart on BlueHost right now, and it's at the optimal response speed of <1s and often ~.5s. It feels pretty instantaneous, which is usually in the .1s range. After 1s people perceive a delay and their mind wanders.