I think all in all, the ad 1500 is the easiest to upgrade. I have been following the hacks among the apex players since 500 series. The 1500 can be hacked via CDR upgrades so you don't even have to open the box. I did it in less than a minute. While for the 500/600 series, you have to know which box you got 'cause there is a lot of design changes which hacks sometimes don't work. I think the 1500 is getting harder because it's so easy to hack. If you have been waiting for this, don't wait too long.
Also, the 1500 hack also takes out RCE protection and not just Macro. I though I also read from somewhere that it also plays Divx. Overall, it's a very good player for the $$$.