I'm looking into buying a premium to grind German ground(and German air but I'm pretty settled on the Ju-288C for that.) And the RU 251 seems like a decent choice.
My german lineup is only up to 5.3- I have the first proper Panther and the first Tiger so it'll be awhile before I have a lineup to back the RU 251 up.
After buying it I'll only have like 300 GE leftover, and pack vehicles aren't an option.
It depends if you like to play with light tanks. You could run into some pretty bad matches because of the lack of protection compared to other alternatives, but if you are used to similar tanks looks like a solid choice.