I don't use it because it doesn't sync chats between devices or have a desktop client.
Full-on e2e is great, and I'd use Signal if it supported my use case, but it doesn't. So I use Telegram instead, as a fast, easy-to-use, grandparent-compatible chat client with sane picture and file-transfer support. The oddball encryption and the fact that it's not end-to-end by default is a downside, but it's better than plaintext and it's actually useful to me, so…
Don't get me wrong, Signal is absolutely the right choice for people who don't need or care about multi-device syncing and only need a mobile client, or people who want the best security they can get. I fully support the widespread adoption of top-tier cryptography, including by people who don't need to protect their communications from global powers. But right now Signal is not (yet?) a one-size-fits-all solution.