Ok might be easiest if I break things down into areas obviously I'm RAF but will loosely be apicable 
Firstly I love it! and whilst I would prefer my DCs not to join up and stay near home etc it is a good life so as long as was not routinely a front line trade I would sign the papers.
Financially
I joined at 17 (ranks not officer) and have always felt a little more well off in comparison to my peers from back home.
Been in 13 years and on £34,000 which I think is a good wage but this can vary between trades and there are two pay bands higher and lower so this is one to research if DS comes home with a list of available trades.
Training
Excellent training and qualifications are available but depends again on trade some courses result in civi recognised quals (I have NVQ l3) some don't.
Lifestyle
Younger
Cash in back pocket, brilliant social, work hard - play hard ethic, lots of banter, travelling the world and a girl in every port 
Older (married/kids)
Vvv cheap housing (but looks it!) my
House costs ..... £80 PCM! But it is nicknamed "the barn" 
Obviously working away from family is hard but whilst away there can be good support and loads organised depends on the place really.
I have made friends that are now more like family and when we go to each others houses we are all treated as family by their mums etc this even includes tellings off!! 
Just a few quick bits as I'm trying to herd my DC out of the door soon. To finish if it was me and my DS joining up I would be very involved in the picking of the trade as it can make a huge difference. The careers office will have certain trades which have more spaces to fill so keep vigilant for being sold something DS does not want.
Research trades on forums army has arse and RAF has egoat so navy will have an equivalent.
Hope this has been of some help 