Old thread, but I guess I'll throw in my 2nl minbet.
A HUD is most useful when you are playing 4+ tables simultaneously, and playing cash games. If you are playing 2nl or 5nl cash games and only have 1 or 2 tables open at the same time, in my opinion it's probably actually better to just observe carefully and take copious notes. SNG and MTT also do not really require a HUD unless you are putting in tons of volumes, which usually also means massive multitabling.
When you start multitabling 4+ cash ring tables, a HUD then becomes extremely useful and is kind of a necessity... particularly when you start to encounter more "regs" and competent players at the 10nl/25nl limits. If you are only playing 1 or 2 tables at the same time, you can probably still make a profit at 10nl/25nl without a HUD, but having one will help you maximize
your profits through self and opponent analysis... and help your hourly winrate since you can then take advantage of multitabling.
I started my first serious foray into online poker HUDless. I was aiming to do a $0 challenge, and decided to only invest in poker what I had won through freerolls
or promotions and later on through my skill. So I set about to buy and use a HUD only if I had a bankroll to support the purchase.
After accumulating a bankroll of about $100 on freerolls, SNG, MTT and 2nl ring games without
a HUD, I downloaded PT3 and started the 60-day trial, with the thinking that in 60 days I would likely have built my $100 bankroll large enough to justify purchasing the micro edition of the software. I was right, and multitabling 5nl and 10nl became a lot easier with PT3 and 60 days later I could easily afford the price of the software with a $300+ bankroll and have enough left over to continue the grind at 10nl.