500 TP per month, extra character slots, more bags, Rested xp. There are a lot more advantages that I could list. If you have it in your budget and you are really enjoying the game, def subscribe. More will be available without you feeling like everything has to be unlocked from the store. You might be able to buy 3 mo for 30 if they still have that.
Characters gain "blue" status for extra XP when they are not played. Log off at night, log on the next day, and you gains some bonus to any XP you earn the next time you are on. You can get the same thing with purchases in the Store, but VIP gets it automatically for all characters on all servers.
As to is it worth it - that depends on you. I think it is very worth it - didn't have to buy all the extras in the store so I can use my TP on other things I want - like Shared Storage, which with all of my alts is indispensable. I hear a lot of complaints from some people who think that VIP should include everything, like MoM, RoI, RoR, etc., but overall I feel it is a value as it stands. It's nice to adventure wherever I want - North Downs, Evendim, Angmar, Misty Mountains, Trollshaws, etc. without weighing whether to buy the quest pack or not. Several of my kinmates have to make those decisions while balancing everything in 3 bags. Of course, they could go premium and buy some bags, and buy those Quest Packs, and buy those extra character slots, etc. It is perfectly possible to buy it all and remain f2p if you are willing to work the deeds and take your time. To each their own.
As to the VIP quest at L20, I have done it once. On my first character to reach L20 (now L50). I generally give in and buy Riding about L13-15 as I get tired of running everywhere. Go figure, though, as I run everywhere in Moria even though I have two goats. I guess it's all a matter of playstyle.
As was mentioned above about the bags and the horse quest...if I purchase VIP and only have one character, will I:
1. Still have the horse and 5 bags available if I choose not to resubscribe to VIP?
2. Have access to the horse and 5 bags if I were to make another character, or is VIP just character-bound?
Even without going VIP, you can buy the riding skill for 95 points per character. If you do go VIP, any toon that is level 20 or higher during the subscription period can do the quest for the free riding skill.
If you buy a horse, you keep it, regardless of account status. And you keep the riding skill.
Any toon logged in during your VIP period also keeps, after VIP expires: all trait slots unlocked, permanent swift travel, no gold cap, 5 bags, 5 auction hall slots.
During VIP, you'll have access to all non-expansion content, so it can be a good time to try before you buy if your toons are high enough level to go to different areas.
VIP is definitely worth it because of how much it unlocks that would cost a lot more to unlock individually (or it would take a very long time to earn by in-game methods). The others have listed what those benefits are.
But note after you stop being VIP any new characters created will NOT have any of those unlocks. So it's best to plan ahead for what characters you want to create during the subscription.
Also VIP unlocks extra character slots, but these will relock when the VIP drops. You can pick which characters to keep active but you will have to purchase the character slot unlocks in the store to reactivate your characters (or go VIP again).
For what its worth, I'm looking at buying VIP status periodically. I just got the Rohan expansion, so I'm waiting to see what is up with that. However, I have a couple of starter toons who were never "VIP unlocked". Once I start playing them a little more regularly, I'll want that extra room, gold etc.