For me it was received very well, except for one foreign PhD interviewer who misunderstood my reason for going into it, thinking that I wanted to learn clinical skills early on. Obviously that would have been worthless; it's not like most of the functional skills you learn as an NA will be useful later on.
For me, working as a CNA was one of my most important experiences in undergrad and I built a lot of my application around it. If you are thinking about working you can talk about how you tried to build your 'bedside manner' and learn how to do stuff like interact intimately w/ very sick and scared people, etc. I wrote about it a lot in my personal statement and have always made sure to bring it up for discussion in my interviews. Everyone except that one guy really really like it, and for that one guy it was just because he misunderstood.