I frequently travel in first on XC on morning and evening peak services - firstly because it's surprisingly good value if you bag advance tickets, in many cases the first advance is as little as 30p extra*, sometimes (bizzarely, and mostly at weekends) it's actually cheaper than a standard advance! And secondly because on these services you'd be hard pushed to find a seat without a reservation, they're always rammed.
The first service on XC mainly offers a little more space, a table at every seat, and light snacks. It's not a huge difference, but when the price difference is so insignificant, I find it worthwhile.
Sadly my only other 1st experience so far has been with London Midland, which is basically different seats and some curtains. Weirdly, I found the slightly deeper seat cushions in LM's 1st to be more uncomfortable than their standard seats.
* I use a railcard, would be more without.