If You want the BEST windows phone possible but aren't interested in the 950 & 950 XL, then the 1520 is the next best thing.
It may be old. But what does that even really mean?
It has only one successor ahead of it, Windows 10 will run smooth on it for many versions to come, and if your not too scared you can replace the battery with a new one to eliminate effects due to age.
As far as how it fairs against rent flagships.. Camera is just as good if not better than many of the latest Android & iOS devices, battery life should be better than most, and it has many modern features that have been gained over the past months (HDR, 4k recording, Dynamic flash, New W10M stuff). There's also Qi charging. It will only continue to gain new features as Windows 10 progresses throughout 2016 also.
The 1520 IMHO will continue to be a great device to use well throughout 2016, but by the time late 2016 comes and the second wave of W10M flagships come, THEN will be the time I feel the 1520 should be put to rest as it will likely have greatly better upgrade options.