This has been debated in other posts however I would like to add my $0.02 again. First for perspective I will state that I used to design and install custom systems that included building and tuning my own speaker enclosures, making major vehicle mods, installing extra batteries, etc. I used to think that any system under $2k was just total junk. I used to like a system that could be heard for blocks, or cranked up out in a crowded parking lot or field to supply an entire block party with high quality tunes. Now I am much older with minor hearing damage and much more reserved.
The upgraded NAV / JBL audio system in my DC Limited is well worth the money. The MSRP sticker price for the whole system was $1650 but after you back out the backup camera, NAV, and bluetooth features then the stereo portion is probably costing you $650 or so. Factor in a 6% discount off of MSRP which shouldn't be too hard to get and your total is down close to $600. Now try to spend this same amount or less for a similar system including 10 speakers, subwoofer enclosure, head unit, amp, all wiring harnesses and adapters, and custom installation service. No way it can be done and come out with anywhere near the quality of the factory upgrade system, plus the factory integrates with the other functions so nicely (NAV rules!).
So the factory upgrade is well worth the money and it does sound pretty good, at least considering what it cost. If you want much better quality and/or more power then by all means do it yourself but be prepared to spend much more and potentially end up with a system that isn't as cleanly installed as the factory rig. Even if you replace certain speakers or individual components the cost of the overall system may still be worth it.