I bought a S&W 5.45 AR upper that came with a Daniel Defense Omega Rail 7.0, Adams Arms Piston Kit, Ranier Arms Xtreme Darkness Flash hider, KAC vertical foregrip, Choate Machine Tool stock (similar in shape, config to an Emod or LMT SOPMOD), CMT iron sights and two tins (and the can opener!) of 5.45x39mm Russian surplus ammo totaling 2,160 rounds. Oh yeah, and 9 of the C-products 5.45 mags. All for $800... thought it was a good deal.
It shoots great, and I've only recently gotten tired of the extended cleaning sessions following each range trip. One day I hosed it down with windex after the range and put off the cleaning for a couple days, only to find some rust/corrosion on the flash hider and inside the piston. That Russian ammo is nasty and needs to be cleaned up quickly.
I'd say a 5.45 upper is a viable alternative as a training rifle thought, depending on your shooting schedule.
Vuurwapen did a review of Spikes 5.45 offer and included a cost analysis, determining it will take ~3k rounds to break even with your additional investment into the new upper vs. 5.56. New, I think it's a good deal, especially since surplus 5.45 is at about 12 cents/round, roughly half the cost of 5.56