What does the software or service do well? Exactly what it's intended to, and the best party for me personally, without being obnoxious, intrusive, or overzealous. Honestly I forget it's installed sometimes. Compared to anti virus/malware products like Mcafee, Norton, Kaspersky, even Malwarebytes, which I have all found to be at the very least from time to time, Bitdefender does it's job and let's you do yours.
What does the software or service not do well? Sometimes it causes issues on my work laptop when connected to an unfamiliar wifi network. Normally easy enough to fix but I haven't found a pattern to be able to prevent it.
What do you wish you had known about the software or service before using? That there was a quiet, background running, non process killing/slowing antivirus available. Too many software models on the market are worse than most viruses you'd be getting without them, Bitdefender isn't one of them.