I have four of these in the 7-gallon flavor ('Reliance Aquatainer'), based on them being the standard choice by my department in undergrad, which takes ~50 students ~3500 miles over 6 weeks of mostly dispersed camping out of vans every year in Field Camp, and apparently they've always been happy with them. Note that the handles slot into a dent in the bottom of the next jug, so you can also stack them vertically if you have them contained on the sides (or don't take curves too fast).
Never any leaks, but I also don't make a habit of filling them and keeping them on their sides, so it's more a testament to the cube design than how good the cap is. I've definitely had a little splash here and there from the vent not being screwed shut before handling, but that's about it.
I also...
neglectfully left them in my yard for almost a year with no observable UV damage (although, they had the advantage of PNW weather and dead leaves to cut down on the UV for 8 months of that). Minor plastic stink/flavor when left to sit for a long time, but 2-3 rinses with hot water and baking soda always brings it back to 'good enough,' and I'm pretty good at picking up even faint plasticky essence.
This is also after trying a number of other solutions, like Reliance's jerry-can shaped (but not standard sized) jug in the same style, grocery store 1 or 2.5 gallon jugs, etc. For multiple days without resupply, these are absolutely my favorite, barring the budget and external mounting points to go up to Scepter water cans.