I haven't made my full thesis on engineering yet, but for those looking to level a profession, I will say it was cheap and easy to get 405 engineering, which allows use of all pvp gadgets and goodies. As far as I see so far, theres nothing to "regret" about going engineering, keeping in mind my other prof. is JC (good income and stats). As far as damage, jury is still out until I get my new trinkets to make up for shifted spellpower, but its not that much different so far.
Opinion Thus Far:
Cost: A+
Fun Factor: A+
Better for BGs/WG?: Absolutely.. I love playing AB now cause of nitro+parachute, I'm all over the place.
Arena: Not sure, I will keep ya posted
Non-Combat benefits?: Convenience and gadgets are much better... but the lack of ability to generate income shows.