I used this for the first time last week.
It's an improvement on the bus, but loads of issues made a very un-smooth journey.
1) Ticket barriers are chaotic. There are way too few of them, it seems like a massive waste of space to have such a large station building but then only have such a narrow/short gate line? There were big crowds backing up at each one as for half of the people (myself included) our valid tickets were not working and 'seek assistance' was flashing up. The need to go through two gate lines in quick succession at the railway station end, doubling that crowd queue and ticket failure is also infuriatingly inefficient. The staff also could not explain why our tickets were randomly not working. Rather than wave everyone through, he was individually opening the gates for each person one at a time. With 200+ people needing to get through. Chaos!
2) Capacity is not enough. Our shuttle was full and standing, packed like a rush hour tube train with nothing to grip, and we left people waiting on the platform who could not board. If it's already this over capacity, it doesn't bode well for plans to expand.
3) Poor signage at the airport. There are seemingly no signs to the DART station in the terminal building, or on the signposts outside. Everything else is signposted; bus stops, car park, taxi rank etc., but DART hasn't been added to any of them. There's also no obvious signage visible from the terminal building exit on the station. Despite arriving by DART and having perfect vision; I simply could not see or find the Airport DART station and had to ask a member of staff (who spoke such poor English he was unable to direct me, but simply vaguely pointed in the direction).
I thought my boycott of Luton due to the bus situation would now be over but, the above outlined how stressful and unenjoyable the DART is. It's a lot better than the bus, but we've gone from a 1/10 to about a 4/10. Stansted and Gatwick remain kings of the budget airline airports.