The TS-419P II is certainly not the most affordable NAS out there, although it belongs to QNAP's lower-end category. Nevertheless after testing it for some weeks I realized that it offers many more features, beyond what average users require, so its price is fully justified. During all test sessions it never hung or froze, even under heavy load, proving that it is very reliable and its firmware is stable. Also QNAP's support is very good, with regular firmware updates and QNAP also hosts a very strong support forum which will be of great help, especially for new owners.
In general I liked the interface and I assure you that you don't need any special Linux knowledge to use it. You just have to know some basic networking concepts and you are good to go, if you plan to use your NAS at home. For small office environments a certain level of experience is required to set up the NAS correctly and efficiently, but I don't think that this will be much of a problem after spending some time reading the manual.
The main downsides I found on the TS-419P II are the weak processor which holds the system back at heavy throughput if the disks are configured in RAID 5 or 6. Nowadays even smartphones feature a dual-core or quad-core processor, so I'd expect a NAS to come with a stronger CPU than a single core ARM model. The lack of USB 3.0 connectivity might be an issue for users who transfer a lot of data by connecting external devices directly to the NAS (according to QNAP all TS-x19P that will be shipped from May 2012 will include USB 3.0 ports). There are also two eSATA ports which offer much higher speeds, so this can be avoided. The short warranty is a bigger concern for a device that works 24/7 and should reliably perform longer than the two year warranty period.

All in all I think that the TS-419P II is a good investment for your money, if you are looking for a NAS device to make your life easier and require access to your files from everywhere along with a dozen other functions. Let's just hope that the prices of mechanical disks will start coming back to normal levels, because currently four 2TB disks will cost you almost the same as the TS-419P II!