For my money, the biggest advantage of AHCI over IDE is the availability of hot-plug for eSATA ports; i.e., you can plug in a drive, and have it 'discovered' by the OD immediately, without having to do a reboot to have it recognized - and, be aware, AHCI drives appear in the 'Safely Remove' list - you must do this before disconnecting, as it flushes the disk buffer from memory to the drive - if you fail to do this, you run the risk of corruption. If you are using an SSD, it also (at least, in the Intel ICH implementation) 'passes-through' seven's TRIM commands to optimize the drive.
As far as actual performance gains (it also enables drives' NCQ - native comand queueing - see the Sorting Out SATA topic in the 'sticky' - I haven't been able to see a significant difference. Here are some thorough AHCI performance figures...
The win boot loader thing is just a mess. It should work, it can work - but, unfortunately, it often doesn't - or takes four tries to get it right! ...And Xp is pretty much the major 'violator' - it has a bad habit of corrupting the boot loader files of later installations (Vista or Seven)! I always recommend using BootIt™ Next Generation - it's fast, easy to configure, flawless, and has great support - oh - and it's cheap! It has excellent disk partitioning tools built in, MBR diagnostics and editing - I run nine OS's off three RAID pair:
I absolutely love BootIt - just the other day a buddy called up who was trying to fix up an older machine, and the installer was only seeing a quarter of the drive. Went over with a BootIt CD; it told me his MBR was corrupted; in all of a minute and a half, I had installed, fixed the MBR, uninstalled - and he was ready to go - and fairly awed