I have downloaded the poker tracker 3 trial version (free for 60 days), i have been using it for a few days and it has proved to me that it can be useful, and as slycbnew' said, especially when you have multiple tables, and i thought it was useful in MTTs coz it seemed to 'remember' players you have already met which is near impossible to do manually in a field of 1000+. Unfortunetly, the trial version does not allow you to create your own hud, so i could not customise to fit my needs/wants, this has totally put me off because the time it takes to look up the stats is often too long. I can understand that i would be able to do this if i purchased the full version, but im still swaying against using it, it is time consuming (compared to not using one) and to me its just all a bit too much, too much on the screen covering peoples stacks or when you move them they cover up the bet lol,and when i use the stats seriously for a session i find myself in too much of a panic to get the info i need and end up rushing my decision or actually timing out. You may say i must be a slow player but i assure you im not, im brill with maths and odds and usually make quick decisions, but when im 'checking stats' it take longer. I still realise the timing issue could be cut by creating my own hud but personally i prefer to just concentrate on the table and use my own info i have collected. I must also say that i will be keeping/using the software but only for the HH ( Hand History, which is exceptionally good) because this is useful to you no matter what. I believe its best to 'do it yourself' at 1 table (only 1), by concentrating and taking notice of everthing on the table, and maybe use the hud when multitabling to maximise your chance of getting the corect info about people.
I finish by saying that it does not suit all, i have done ok in poker without it and i hope to do better by using it for the HH, and possibly using the hud in a big MTT (but i would only turn it on when it was late stages), but the hud is just not for me.
If you havent yet, try it out! GL at the tables!