In all honesty, there's very little to improve on the M10 (save for the lock switch and/or battery door) and Sony hasn't made any mention of any refresh/update/new product so I don't see why the OP's afraid to make the jump. The M10 has held its value, anyhow, and even if Sony was to release a successor, you can expect it to run upwards of $300 at the onset.
The backlighting on my three year old unit's pretty uniform.