Couldn't get on with the 5610u but love the 5000u
I thought I'd post this for anyone who is wondering if the GW-5000u is a worthy upgrade from the 5610u.
I've had the 5610u a year and was never completely happy with it. Features and looks wise, it's fantastic, but I was between holes on the band and the strap never felt that comfortable to me, even though it broke in a bit. It would sit off my wrist on the looser setting due to the lug angles. I tried a combi, but that wasn't for me long term.
I still wanted a square in my collection, so took a chance on the 5000u hoping it would be an improvement.
For me, I can say it definitely is. The strap is noticeably more supple and comfortable, and the extra weight (which may be a disadvantage to some) means it sits more firmly on my wrist (7.5") without moving around. The display even seems slightly better at angles than the 5610u (might just be me though?), and the black minimalistic look is slick.
I think if the 5610u fits you and is comfy, you won't see a huge difference in upgrading, but if the 5610 niggles you then it is worth it. This and the GD-350 will be my primary G Shocks (for now).