Here is a good article about the pros and cons:
http://www.medhelp.org/user_journals/show/144600/EVIDENCE-BASED-APPROACH-TO-NEUTER-SURGERY-AND-INTRODUCTION-TO-EVIDENCE-BASED-MEDICAL-PRACTICEMy American cocker was neutered at 14 months old. I wish I had him done earlier as his aggression towards other dogs started at around 8 months old. Although it got better after being neutered (and being put on thyroid medication), some of the learnt behaviour remains.
Before he was neutered, he started off by humping me, then eventually other people, then female dogs and eventually every dog he met and would let him. When he was neutered, this stopped completely - and I am the only one left that gets humped once in a while when he is over-excited.

So yes, that can work really well in that case and will probably with most dogs if there is a problem in that respect.
Jesse's recall also started going out of the window from around 8 months old despite a lot of training. Once neutered, he recalled just fine again almost overnight.

I know of several dogs in my classes that were distracted by everything going on around them before being neutered, once done they have no problem focusing on a task at hand.
So yes, there can be quite significant behavioural improvements, but that's by no means a guarantee and therefore shouldn't be the only reason to have it done, though it can be a contributory reason.
In some rare cases, aggression can be increased in more insecure dogs so if a dog is very nervous as it can have adverse behavioural effects. It is also important to remember that aggression can sometimes be temporarily increased after neutering due to the sharp drop in hormones, but if handled correctly, they usually return to normal behaviour after a time.
All my dogs are neutered regardless whether they are male or female, rescue or not. With my dogs I haven't seen any adverse effects in any of mine (5 so far) and I haven't so far seen any adverse effects with early neutering either (before maturity: Skye was neutered at 5 months, Flash - my saluki - at around 6 months and my lurcher Dylan at around 1 year).
My American cocker's coat probably got thicker, but it is by no means unmanageable, though it probably lost some of its shine on his shorter back coat. Skye is 7 months post spay and so far, her coat is still as glossy and smooth as ever, hopefully it'll stay that way.
