1) Is BMI a good airline to move my loyalty to? I've been happy in the past flying with them as part of *A.
Generally everyone seems very positive about BMI's program, I'm certainly happy with them, and if you fly up the front the earning rate is very generous. Being able to pay for half an award with cash (called cash and miles) is another great benefit. Also, once you get silver you get a 25% status bonus on your miles flown on any *A flight.
2) Can i easily use other *A carriers to fly from Australia a) to Europe, b) to the US (and still collect status miles)?
*A Carriers offer very good international service to Australia. You can fly UA or NZ to the US, for Europe you can fly TG or SQ. However, there's no domestic *A carrier since the collapse of Ansett. There are persistent rumours that Virgin Blue will join *A, but so far they're just that, rumours and they may be wishful thinking.
3) Relative to the competition, are BMI (or code share) prices reasonable (purchased from within Australia)?
As far as I know, there are no BMI codeshares to Australia and they certainly don't fly their metal down here. However, as I said before, you should have absolutely no trouble with international flights on *A from here.
4) Will i have similar kinds of status options that i currently have with UA?
Status works a little differently on BMI, when you get to 3000miles, you go to Blue+ level and the counter resets to 0, also your membership year resets, then you need to earn another 16,000miles to get to silver, then your counter and membership year resets again. Another 38,000 and you're at Gold. Think of it as needing 57,000 to get to Gold, then 38,000 to renew.
5) Do people generally have positive things to say about BMI's customer service (especially when problems need resolving)?
There are two call centres, the UK one is great, the Indian one... well, it's an Indian call centre.
... or should i just stick with UA?
Only you can answer that one, have a read around this forum and compare what's on offer to what you currently get and what your future flying patterns will be.
Hope this helps.