In my experience, the quality of your actual ADSL service is fairly consistent from ISP to ISP, and is more dependent on factors like your router, quality of internal house wiring, quality of the phoneline to the exchange, and the distance to the exchange. The ISP can't really control any of that, and 99% of speed or sync problems are caused by that.
Customer service and price are where ISPs are set apart - TPG is renowned for not having great phone support, but their reps on Whirlpool.Net are great, and their price is pretty good too.
I was with iinet for 3 years paying $90 a month for 200gb/200gb Naked DSL, and now that we've moved house we're going onto TPG's ADSL2 + Homephone Unlimited plan for $60 a month. I can live with poor phone support for unlimited downloads and a homephone for $30 a month less.
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