I've only got a 720p 32" TV but the clarity of things like Planet Earth on Blu-Ray over SD broadcast or DVD is stunning.
The format war is over, the rental places have loads of BR, supermarkets and online places are doing offers that mean you rarely have to pay more than a tenner each, and you can pick up a decent player for £130ish.
If you've got (or are planning to get soon) a HDTV then it's a no-brainer. Have a look around the back of your all-in-one system and see if it can take inputs from other sources – if it can just hook up your new BR player and use it just like an AV amp.