I use TPP, mostly because they've been pretty easy when it comes to keeping multiple domains under control - easy to delegate to default nameservers, pay by default credit card on file, etc. 95% of the domains I look after (60-70ish personal ones and then others for friends and a bunch more for clients) are .com.au though and I keep the majority of my .coms with TPP also even though they're more expensive - just saves having different logins for every domain with Joker, Gandi, etc.
I guess it makes it a bit easier to charge clients a fraction more (say $50/2yrs) via the same registrar and they don't necessarily feel like you're screwing them.
