We have been using Site5 for past one year, mainly for our sites under development or beta stages. While their speed is not the best, it is still not the worst. Definitely we could not find it lightning speed as they have named their best plan. 

One thing we liked about the plan was that you will get a separate cpanel for each of your sites. This will help you a lot if different developers are working on different sites.

Also, one thing we liked about Site5 was that, their ticket based support gets back usually within 24 hours max.

Overall a good experience, but somehoe not good enough for a site with large number of users.