What does SiteGround do well? If you're looking for a reliable hosting partner with lots of features and a convenient price, I think you must consider Siteground. They have a really skilled and highly responsive customer service.
What does SiteGround not do well? They are about to abandon Cpanel. They will offer a customised backend dashboard/panel. Many users will be disappointed by this move, but until we see the new backend it is difficult to judge.
What do you wish you had known about SiteGround before using? There are a few interesting features that are not included in the lower hosting tiers. Keep an eye on the backup options, for instance. It might be worth it to buy a better plan just for that.
Have you used a competitor to SiteGround? If so, how does it compare? I've used 1and1 and aruba. Siteground is way better than both. Most of all, all the support staff I dealt with were extremely skilled and helpful.