I'd count him as one of the best military sci-fi writers around, pity he basically only writes it inside the Warhammer settings. Kinda like Michael Stackpole, great mil-sci-fi writer but you're unlikely to see much of his wrok because it was all in Battletech and Star Wars, which kinda limits exposure even while increasing it.

Comparing First and Only to his more recent works, I'd say it's a lot like Pratchett, Colour of Magic and Dark Side of the Sun versus Men at Arms or Fifth Elephant, you can really see how much improvement even very good writers can make. Abnett's 'Legion' is probably his best work to date, IMHO, and I'm a pretty big fan of the Gaunt, Eisenhorn, and Ravenor series.