I've used Serve-U and Bulletproof. Serve-U was a nice app, easy to use, Nice interface, lots of options and features although nothing that really stood out to me. I use Bulletproof now, I don't like the interface as much but there are 2 features that I do really like.
1. You don't have to order your directory permissions, the program looks at them all and decides the order automatically. Not a big deal I know, but when your changing permissions, creating accounts etc etc etc all the time it's just one less hassle to worry about. Serve-U on the other hand you had to put the permissions in the correct order, otherwise you could end up locking out or allowing people into stuff they weren't supposed to be.
2. You can create nameless accounts, so that if you have a group of people that need access then you can create 1 account for them and have one password for them and they can login in with any username you want. So if you have Jim, Bob, and Joe in accounting, you can create a log in so that each person can use his own name and the same password as the others. I guess it's kind of one of those features that either you use or you don't, but I have found it quite handy.
James