Unless you are going to go nuts on the engine, ie forced induction/hardcore NA, aftermarket standalone ECU's aren't worth it.
They are only really suitable for hardcore NA if you are going to use high lift/long duration cams, quad tb's where conversion from MAF to MAP sensor is essential, high compression etc; for forced induction to control larger injectors, alter timing under boost etc.
Piggyback ECU's however are a different matter. Myself and Noss run a unit from Perfect Power called the SMT6, and they will be tuned to provide timing adjustment/increased fuelling with the turbo setups. They work by intercepting and modifying signals going to the std ECU, and can do such things as drive extra injectors, activate a switchpoint (ie shift light for one example) etc. The unit is worth about $590 from Powerchip in Sth melbourne and i think shits over units like the Greddy E-manage which is twice the price, which i was going to get initially.
Their site is
www.perfectpower.com, software is free from their site.
If you can't tell, i really like the SMT6.