Both you sir, and Harm say this 5.45x39 is sweet. What exactly about it besides the price? Is it ballistics or just a fun factor?
D. All of the above.
Right now 5.45x39 costs roughly half what 5.56 ammo costs. However, since all of it is imported from Russia that situation can change at any time. When I was shooting lots of it, the price didn't matter to me since I did not have to pay for it.
In terms of ballistics, the cartridge has much going for it. It can be a very accurate round, but is limited by the rifles available for it and bullet selection. The AK is not praised for its accuracy (though it is more accurate than people give it credit for) and the only bolt action rifle I have seen for it was an East German rifle that was very poorly made and was generally less accurate than a decent AK. In my opinion, the 5.45 has better terminal ballistics than the 5.56 does with either the 55 grain or 62 grain military loads. There are many hunting and LE bullets that far surpass military 5.56 ammo as well as 5.45 ammo. The 5.45 is limited by the fact that it is only produced with FMJ bullets. Compared to 7.62x39 it does have less penetration but still does well in ballistic performance (until you compare with Hornaday 7.62x39 loaded with their V-Max bullet).
5.45 also has low recoil. Even a small, lightweight gun like a Krinkov is easy to control on full auto. In semi-auto, fast double and triple taps are no problem.
Finally, it is something different and uncommen, giving it a certain cool factor.