Well, all I can say really is that if you want to try it then backup everything, regularly (as in daily) because the day will come when either your site gets popular or they have problems and you won't be able to access your site or they will block you from it.
At that point you will need your backups to move somewhere else. I'm speaking (writing) from bitter experience here having had an account suspended on bluehost for too much SQL usage and a wonderful 36 hour down period a few weeks later.
I am now the most paranoid person you are likely to encounter when it comes to redundancy and fall back plans, and once you have people depending on your system being accessible or you have people paying you out of their hard earned cash you just can't take these chances.
At a minimum (when I'm not testing things as I am at the moment) I probably spend just over double what wehican does per year (about $300). For that I get:
* A VPS server that I can configure, tune and abuse to my hearts content. I take daily backups of the entire system so that should something happen to the server then I can restore the entire system (MU and all content) on a new server in minutes.
* External DNS - My DNS is handled by two external companies so there have a total of 8 DNS servers that have the details of my sites at any one time. (I usually only use a single company at a time - so 4 will be active, the other is a backup DNS company). Both have short TTL (Time to Life) settings so if I change an IP address to point to a new server it starts resolving in minutes rather than hours.
* Email doesn't touch the servers, it all goes via Google for your Domain, so if the servers are down for a short period I can still send and receive emails. Vital to keep customers informed. I have an autoresponse set up on a status email so that customers can send a message to that account and it will reply with the current system status. The message is set manually.
* Backups - as well as the VPS backup above, my databases are backed up hourly and pushed to Amazon S3, Client uploads are backed up to S3 daily (in between server backups so I effectively get backups every 12 hours). And as an absolute failsafe I also have an Amazon EC2 instance image of my WPMU server setup so that I can get that up and running if the company providing my VPS has problems with it's internet connection.
There are a few other things but I wouldn't want to bore you anymore :)
And I have 6 months with Bluehost to thank for it all :)