I think her journals are fairly interesting, but I found myself skipping some parts - saying that that it was a long time ago since I read this. I think someone who doesn't read much might be better off with a biography and move on the to journals if they like it. A lot of the stuff in the journals is rather self obsessed, but what else can a diary be. Sometimes interesting, sometimes frustrating, occasionally dull. Anyhoo thats just my opinion
I wouldn't recommend it. I read The Bell Jar, loved it and was hoping for something .......... I don't know, just more deep and as you say profound. But couldn't get into the diaries.
I haven't read the diaries but loved The Bell Jar. Very atmospheric, subtle and understated. A similar read is a book called Girl, Interrupted by Susanna Kaysen. It's also been made into a film which stars Winona Ryder. Both are great, sorry I've strayed off from your question quite a bit, but it really is worth checking out as is The Bell Jar.
Don't wish to sound rude, but if I were someone who didn't read much of anything, and wanted to start reading - I wouldn't start with Plath. I'm not suggesting you are intellectually incapable, but I think if you're not used to reading, you may find Plath a bit heavy going, although as mentioned above, The Bell Jar is a fantastic book.