To answer your question Pete, they deploy a topology for you give you the keys and leave you to it, aside from a service catalog of requests as define here; https://kb.sitecore.net/articles/133931
Bic's response is accurate, it isn't "managed" (put yourself in a licence holders shoes on this definition) and Sitecore doesn't deliver any real-world valuable SLA for your website speed, security or uptime. In some cases the multi-tenancy nature of their management model would need scrutiny for anyone with PII, PCI or equivalent data. We have two Managed Cloud customers we have inherited and we are inhibited as to some the things we can do with them. We maintain separate Azure Subscriptions alongside for things like Functions, APIM etc. at a cost to us.
The contractual model won't give you the elastic nature most customers are looking for. It is fixed price, fixed term, with arguably significant markups and overages are costly (I know this first hand), although in recent times, I have seen Sitecore be grown up about this issue with pooled contracts. You cannot fault the support teams delivering the services, they are clearly skilled and always willing to help, but there is only so much you can do, and to what extent this goes beyond a typical partner skills, I don't really know.
In terms of DevOps, deployment, management, there is none. 100% bring your own and the pre-defined topologies given don't support blue/green or geo-scaling. For the customers we inherited, we have had permission from support to delete the whole topology and deploy our own. Which we have done to achieve what we believe is the first class approach, whilst they say they don't support the custom topologies, I would argue how they were supporting the existing ones, when if the site goes down, it's a finger pointing exercise back to you. At the end of the day, its not an uptime SLA and the customer needs to be clear they need you as a partner still to manage that.
I would also add that most Sitecore partners are Microsoft partners, which affords you the benefits of Microsoft Support. Issues I have had before are Microsoft are typically at arms length to you through MCS, meaning you cannot leverage Microsoft Direct support for these resources, you need to go through support (other people may have had better experiences here, but when complex Microsoft issues happen, they have directed me to Sitecore first).
I have not used Rackspace, so cannot confirm how this truly differs, I suspect its the SLA aspects.
My advice, do it yourself, or work with an experienced Microsoft/Cloud/Azure/DevOps partner, full disclosure, I work for one of those and there are a few of us, but my experience of the quality of skills in this amazing community, as long as you can satisfy your customers requirements to manage the top-to-bottom stack, follow amazing blogs in the space, refine your topologies to meet your needs and can procure Azure through CSP or the clients existing EA, you would be better off in the long run.