I think the answer to that would depend on exactly what you are looking for in a host. Some of the primary reasons I switched to DreamHost were as follows (no particular order);
The ability to host multiple domains/sub-domains with a large degree of independence, by this I mean the way DreamHost treats sub-domains just like full domains, with their own web accessible directory, email accounts etc.
DreamHost has an attitude that I like. A good example was during a particularly bad period of network problems last year, instead of trying to gloss over the problems, DreamHost did their best to keep us customers informed as to exactly what the problems were and how they were being addressed. The problems themselves were annoying, but not nearly as much as they would have been had we been kept in the dark.
Then of-course there is value, in my opinion nothing comes close to offering the value of the DreamHost plans.
Well, it depends on what you mean by 'medium sized' sites, but I imagine you will be fine on even the Level 1 plan. You can always upgrade at any time via the panel, there is no downtime when you do this.
I am assuming site.com is your domain? If so, then yes, you can choose whether to transfer the domain registration to DreamHost and host the domain here or leave the registration where it is and simply configure the domain to use the DreamHost name servers.
DreamHost use a custom written admin panel, which can be a bit of a culture shock if you are coming from something like cPanel. Many things are done a little differently here, but once you get used to where things are and how they are done, the panel is a joy to use.
Feel free, we'll do out best to answer them
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