I have a S&W 5.45x39 AR carbine gas upper on a spike's lower.
The only thing that sucks is cleaning - not the actual cleaning but that it must be cleaned after shooting. Otherwise, there will be rust.
I am on my 3rd 1000 round can of corrosive ammo. I have found some pitting on the bolt but not much to worry about. It is accurate and reliable (with heavy hammer spring it came with or Giselle trigger that is has now + spikes ST-2 or ST-3 buffer). I like the feel of 5.45 more than the 5.56 AR when shooting. It feels milder than the 5.56.
If cleaning and cleaning right after shooting are not issues, cheap 5.45x39 corrosive is a good thing. If you shoot a lot per range session, then you will love the 5.45x39 for the money saved compared to 5.56.
Keep in mind that steel case non corrosive 5.45x39 cost the same as steel cased non-corrosive .223. So, saving will only come with shooting corrosive ammo.
I have a carbine gas because the mid lengths wre not available yet when i got mine.