We are coming into London from Shannon on May 28. Our flight is scheduled to arrive at 2:25 at Gatwick, and we'll take the express to and then a cab to our hotel Millenium Glouster. Since the day is almost gone, I thought we'd go to the London Eye after we check in. My question is: should we reserve the tickets ahead of time, or even buy the fast track because it's a Saturday? Could we comfortably make a 7:00 pm reservation, barring train, cab, check in problems? It is our first time and I don't have a good handle on traffic, distances, etc. thanks!

If everything went smoothly, you could make a 7 p.m. ride, but it might be tight. I think I'd be tempted to make my destination for Saturday night a walk in Hyde Park and a nice dinner -- there are some good restaurants near the hotel.
If you get settled in your hotel comfortably early, you could take the Circle or District line from Gloucester Road to Westminster, cross the bridge and check out the line at the Eye, and if it's too long, walk around Westminster checking out the Houses of Parliament, Westminster Abbey, and the Horse Guards.

If the flight lands on time, I see no reason why the OP shouldn't be in their hotel by 5-5.30pm. This is a public holiday weekend in the UK, though, and there will be plenty of people travelling through Gatwick: whether this adds to ground congestion delaying planes getting to the stand, as well as people-crowding (mainly for departures rather than arrivals) in the terminals, I'm not sure.

If you don't sit down on arrival at your hotel, but turn straight round, you could be at Westminster tube by 6.15 - 6.30.

The holiday weekend might well bring out more people who want to take the Eye, but at that time of day, local families will be wanting to get the kids home, so demand may have balanced out.

But as vcl says, there's plenty to see in Westminster if the lines for the Eye look too long - and St James's Park for a bit of peace and quiet. Or you could stroll along the South Bank, check out the bookstalls and the programmes in the various halls and theatres, stop for a bite, and so on.

Thanks, that makes sense. We don't want to be running blindly around.

Personally, standing in line to check in for a flight, then standing in line for security and ID, standing in line to board and then get off the plane, well that's enough standing in line for me for one day. IF it's a beautiful day, I'd call the Eye and make a same day Fast Track reservation. But some people are more patient than others (i.e., more patient than me).

I considered reservations for an upcoming visit, but.... I don't want to pay for it and have it be one of those cloudy days in London where you can't see 10 feet! So I am just going to chance it!