I've used the Gatwick Express service for 3 return trips and one single from Gatwick airport to London Victoria now and have very mixed opinions on the service. It is convenient that the ticket is open (my only reason for using the service). This means that if you are flying into Gatwick airport and have any delays on your flight you will be able to jump on any Gatwick express train whereas pre-booking some cheaper trains may limit you to one specific train time and if you miss it you will have to purchase a new ticket.

This happened to me on one occasion, when flying from Gatwick I got stuck in traffic getting to London Victoria due to a Syrian embassy protest and a 15 minute bus journey took 2 hours, 2 buses, a walk and a train. Luckily I had left plenty of time to get something to eat etc. so I made the flight but missed the train and had to buy a new ticket.
On this occasion I used the Gatwick express and as soon as I got on the train I was told there was a delay (not what we needed when we were already late), luckily this was only a few minutes and we got to the airport in time.
THE SERVICE
The Gatwick Express is a train goes directly from Gatwick Airport to London Victoria. A train leaves every 15 minutes and takes 30 minutes except on Sundays when it takes 35 (I'm not complaining over 5 minutes but I wonder what the reason for this is, is there a slower train speed limit on a Sunday?).
The train station is in the South Terminal of Gatwick airport, if your flight is in the North Terminal there is a free shuttle bus to take you to and from the South Terminal.


THE PRICE
If there's a group of you travelling you can get 3 or 4 tickets for the price of 2 which makes the service much better value. You can't get the group offer by paying on the train so either get your tickets online or in the ticket office.
If you have a railcard you are entitled to 1/3 off the ticket price
Another "great offer" is that you save money by booking a day return, which is useless to most commuters as it can only be used after 9.35. Nice one.
Another "great offer" is that childrens tickets are half of the adult fare BUT only for children up to the age of 15. So if you're travelling with 16 year old children they are classed as adults.
Prices:
Anytime Single - £17.90
Anytime Return - £30.80 (lasts one month)
Day Return - £23.80

If that's not pricey enough, opt for First Class and blow a few more pennies:
Anytime Single - £26.00
Anytime Return - £50.00 (one month)
Day Return - £30.00

This is the part that annoys me, I think it is a lot of money if you are travelling alone or as a couple, group discounts are available so it isn't too bad if there are a few of you.
The First Capital Connect service offers a similar service from Gatwick airport to Kings Cross costing only £18 return for only a slightly longer trip
Other trains where you are restricted to a certain time can be bought from only £5 each way, if you miss the train for some reason you will have to fork out for a new ticket though.
For those who use this route regularly you can buy a Carnet which gives you 10 tickets for the price of 8 and are valid for a year.
ON THE TRAIN
The trains are pretty basic, due to the fact that they are fairly frequent I've found them pretty quiet during the day but I did get on one which was overcrowded but this was at peak time for commuters and I expect all transport would be busy at these times. There isn't as much luggage room as I would have expected for a service which runs from an airport.
The toilets have always been clean, as have the trains in general, when I have used them.
There is no refreshment service on this train, which I wasn't bothered about as I didn't expect one for a short journey.

THE STAFF
Every time I have used this service the staff have been appalling. They scrutinise the tickets when you get off the train, trying to catch tourists who've got on the Gatwick Express instead of another Gatwick to Victoria train not realising that many trains operate the same route and charge them. I've witnessed this whilst trying to get out of the Gatwick Express turnstiles on numerous occasions until one day it was my turn. I accidentally gave them a Kings Cross to York ticket on one occasion (I had several train tickets in my purse for various journeys that weekend) and the SCREECHING that went on from the member of staff was unbelievable! I apologised for being disorganised and told him I'd locate the correct one and got a big over exaggerated tut in response. I did what I usually do when I experience people behaving like idiots and started laughing, this seemed to anger this particular member of staff even more. I tried to tell him to calm down, it was all going to be okay and I would find the correct ticket immediately and this just made things worse!
He started ranting and raving about how people were always trying to "pull a fast one" and I just continued to laugh. I found the correct ticket, he threw it back at me and let me escape the through the turnstile before picking on someone else!
I won't make that mistake again! This was one of several occasions where I have seen the staff being rude to people. On my first ever trip to Gatwick airport I asked a man in a Gatwick Express coat when the next Gatwick Express train was due and he told me to "look at the board" and walked off. The board wasn't working. I'm not simple.

OVERALL
The service seems efficient, it is frequent and has been reliable whenever I have used it. Other than a bit more luggage space needed the trains are fine. The cost and staff put me off using the service again though. When there are cheaper trains services are available I'm always going to choose them so unless the Gatwick Express is going to reduce their prices I can't see me using them again.