Playing so far (level 27 is the highest), as I'm a gatherer, I spend less time on my mount than not, but I do like the extra bag slots I've gained with the Draft build of the Imperial horse.
The extra storage (I'm up to 110 with bag upgrades too), makes life much easier for longer questing/gathering sessions as I have an abundance of crafting toons and I don't pass up a crate/bag or crafting node!
At launch when I found out about the feeding element, I wasn't sure which way to go, speed, stamina or more storage, so finally settled on buying two mounts to start for the four class characters I'll be leveling. A speed (light) and a carrying (draft) horse. The intent is the speed horse will be used in PvP or opening a map traveling when I know I'm not gathering/questing out in the world and the draft horse is the regular mount for the extra storage.
With the Imperial mount, the cost was 1g for each mount, plus 10g for the extra stable slot. Feeding is on a 20 hour timer, and costs 250g for either apples (speed), hay (stamina), oats (carrying capacity). So, I'm paying 2000g a day to (250x2x4) to keep the horses fed for the four main characters I'm planning to level. It takes 41 days (every 20 hours if you don't miss a day), to max out your horse skill points.
You can also rename your mount, although I don't think you ever see the name except when you visit the stable to feed them. I used one name for all the speed horses (Dash) and one for all the carrying (Ringo), two of my real life horses, just to keep track of which is getting fed what.
There is a 'normal' mount that can be purchased for 17,200g while the other mounts in the game cost 42,700 gold and start with better base stats, so will have a slight edge IF you feed them up. The extra run speed on the Light Horse will be handy in PvP, and I'll likely buy one closer to max level for that purpose. But I couldn't see being mount-less during the leveling time. So the set up I have works fine for me, even if it may short me a tad later on.
Here are some pics of the changes at level 20 compared to the base horse.
Level 1 Imperial Horse
Level 20 Imperial Horse (Light/speed) 36% run speed
Level 20 Imperial Horse (Draft/carrying capacity) 20 extra bag slots
This does confuse me, as I'm not sure if my horses are getting bigger or they have their size adjusted to the different races. The High Elf I'm playing in these pics is on the smallest setting, and the Wood Elf next to me, well his mount is substantially shorter and smaller!
Tamriel Journal has a wonderful overview of the mount system in ESO that goes into great detail how the mounts work and the variety of the ones available ingame.
The only two things I do have issue with in ESO, as a real life horse owner, one if I had a horse with back legs like that, it would likely be unable to walk let alone run from being crippled. Two, whichever researcher worked on this, 99.9% of the time you mount a horse from the left side, not the right! Small detail!