Engi is actually a great 1st choice since the reflects and extra pet dmg in the high levels are great.
Also if you can get the 200 droid on at a decently early time like 90-110 you can score a lot of XP soloing hecks. So its like your own team all the time
Key is really to have good nano skills (Steppes of Madness cloak - called Neleb), De'valos sleeves. etc.
Also of note is engi's are dirt cheap. The gun is just upgraded through time (check
www.ao-universe.com) for the solar powered engineer pistol.
Also you get get around in OFAB/Jathos/Reactive all freeeeee!!!
If you are still in the experimental stage though I would rate them as as follows
Desired professions
Doc - cheap too. Both end game guns and armour are Free nodrop stuff
Enforcer - A bit more expensive as you will need Alien armour
Crat - can be cheap. 1 gun is no drop and can use cheap armour drops
Soldier - expensive (sort of) OFAB is very good and levelling you can use the Kaehler sleeves
Semi useful
Shade - mega DPS, but tricky to solo and can be hard to get he hang of. Moderate costs
Engi - DPS/Auras/Reflects, can be dirt cheap as long as you keep bots high level
Fixer - NCU, runspeeds and evade and high DPS, moderate costs and very strong in PvP
Keeper - Auras are nice, extra AAD and DPS. moderate costs and nano skills are annoying. Can be good in PvM at TL7, but tricky earlier
Not very useful
Every thing else. In fact things like Trader/Advy/NT are all fun to play but they dont specifically offer anything in the team that another "core" prof cant.
Its sad to say but you will go further and faster if you use a desired profession 1st as you will find teams easier. BUT AP will help you with all professions and if you like engi, its a good 1st one for sure!