I also have a Synology, and review Synology units for the company. I've had a NAS for over a year now, with no problems whatsoever.
The best RAID to use on the unit by far is Synology Hybrid RAID (SHR). It's in essence the same as RAID5, but it's easier to upgrade disks to a larger size, and it will use the full space on each disk, even if the ones in the array are different sizes - standard RAID5 will only use the same amount of space on each physical disk, depending on the smallest sized disk in the array.
As someone said above, RAID5 is a fail safe. It's not a backup. All it does is save you should a single disk in the array fail.
The format used is EXT4, which is standard for most Linux based systems. You can use all MAC services, such as Time Machine, iTunes server, and so on, perfectly fine. The MAC will find the NAS on the network straight away automatically and list it in Finder for you - very easy.
Take a look at my YouTube channel for lots of videos on Synology NAS. The ones that may interest you in particular are the ones where I look at the DSM management software, and the backup software built into the system. I also review the mobile management app - where you can use an iPhone or iPad to completely manage the device.
Synology have just sent me a DX510 expansion unit, which attaches onto the initial NAS via a secure eSATA connection, which can be used to extend your current volume, or backup your entire NAS. I'm reviewing this at the moment, so keep an eye out for the video, which should be up within the next week or so - you may find that useful when it comes to learning about backing up etc.