I prefer C&S for the majority of parcels I send. I find it convenient, and I like the fact that I can pay for postage as I go with PayPal, which effectively reduces my direct out of pocket expenses. 

The saving varies depending on the other AP services you compare it to, but using the 500gm rates and not taking into account any packaging (including the C&S satchel) and printing costs...

$6.95 for both C&S / over the counter rates - C&S provides unquestionable proof of shipping if scanned and at no further cost or effort (though keeping copies of lodgement receipt / address labels via PDF docs is recommended), over the counter doesn't (the tracking number / receipt may help if needed, but IMO I'd rather just know I had the proof than hope/convince I had it).

If you want the same proof of shipping C&S provides when posting over the counter, you need to add $2.95 for signature on delivery, or possibly arrange with your post office to stamp lodgement forms, if they are agreeable (mine won't stamp official lodgement forms, but they said they'd stamp a handwritten name & address on the back of a receipt - couldn't be bothered doing that every time I go to the PO, though). 

Information about what SOD includes varies, with C&S it currently provide up to $100 compensation, while over the counter insurance is advertised as an add-on at $1.50 per $100 (or part thereof), although that has been questioned, if it's true, to get exactly the same service and $100 insurance, C&S = $9.90, over the counter $11.40.

C&S flat rate satchel can't be insured beyond $100, over the counter can.

I occasionally experience problems with the C&S site, usually when there are service updates or site issues affecting everyone - which do admittedly happen a bit more frequently than you would expect. At all other times, it takes about 20 seconds to process a label, a few seconds longer if I'm using an address not associated with the linked eBay ID, then about a minute to pay, and a few seconds to print. I'm not sure why I don't seem to experience the issues many others frequently do, but if it helps any I use the latest version of Firefox, Windows 8, Adobe XI.