I'll start by saying I'm not a fan of the subscription model and subscriptions in general, I tend to forget I have them and feel guilty if I don't use them, it's kind of a lose-lose situation for me. But I want to go through my experience with this package (lol) and you can decide for yourself if it's worth for you or not. Depending on your progress in Idle, the subscription value can vary greatly, I'll add my thoughts on that as well, along with some suggestions to get the best value.

First of all, I think everyone should try Idle Premium at least once, it's a great value pack with the 1st time purchase, 1k coins and x5 of every Idle Power on top of the Premium benefits, if you love Idle it's hard (lol) to resist. It's also a great way to support Warzone. With that said...

Having reached the plateau after 250%+ AP, Premium sped up my progress only by a factor of 2 or very close to it. The biggest contributor was 'faster hospital upgrades', everything else had little to no impact on the game, with one notable exception. But let's go in order, from most to least useful:

1. Hospital Upgrades x100 faster
This is the best feature and the one worth 80% of a premium subscription. You can get instant hospital upgrades from the 1v1 ladder, but it's really hard to get to that level.

2. Tank + 12h Idle Time
I'm grouping these two features together because that's the only way I used them. The tank is simply too slow to be useful in any other way. You set its path before going to bed and wake up at a market or a cache big enough to get you to the first market. For me this happened once every Superascension, so not very much.
The real value of the Tank+Idle Time is in the Post Superascension levels. Specifically, all of them except the last 3. Those are impossible to insta cache-hop (even with Insane AcacheB+CB and maxed Phase 4 advancements) and notably hard and time consuming to raid for 'Skip Level' Powers. But if you enable 4 or 5 multi level advancements and set the tank on all of those levels, in 8-10 hours the tank will gather those powers for you, or get you to a big enough army cache to start hopping around. This ability alone is what makes the Tank+Idle Time valuable.

3. Stand-alone Client
I am already a member and have access to the the Stand-alone client, this was a non-feature for me. But it's good value for those who are not members, since Idle runs much faster on the client than on the website.

4. 20% Faster Digs
I can't dig enough artis to merge into a Rare before I Superascend, so this feature did very little for me. However, if it takes you around a couple weeks to Superascend, then you might have enough time merge your digs and bring the arti along.
Pairs well with 30% faster dig from the CW rewards, as the total dig time will be almost halved.

5. No ads, 20% Bonus, free spin
I don't mind the ads and I play the game enough to get my daily free spin. The ability to switch the 20% Bonus was useful from time to time, but never made any tangible difference. These features are mostly about comfortability, with almost no impact on the game itself.

6. Insta-Mortars
Accurate mortars (with the 1v1 ladder benefit) can cut 1 Hospital Upgrade on a handful of levels and speed up your progress. However, you already have insta-hospital upgrades, so this feature is kind of redundant and rarely useful. Made no difference for me.

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Depending on where you are on your Idle progress, the subscription can be more or less valuable. Here are some thoughts and suggestions:

a. Pre-Ascension
If you never ascended, you won't be able to exploit the full value of a premium subscription by a long shot. If you really can't wait, then at least make sure you reached Rise & Fall and have an Epic AV. Use the tank overnight to reach a market and clear one level per day. Regardless if you have Premium or not, remember to exploit powers to help clearing levels quickly (use them but don't waste them, there will be plenty on the maps and you need them now more than later).

b. Post-Ascension
Not suggested as well, as you still need to unlock Phase 4 and likely have trouble getting levels started. Wait at least until you have the Skip Level Advancement to 4 or 5.

c. Pre-SuperAscension
With Skip Levels maxed and Phase 4 unlocked (or close to it), this is the best time to activate premium and get the most out of it. You'll need to chain-ascend at least half a dozen times (less with the CW AP reward) and replay some hardened levels every time to make enough APs to Super Ascend (suggestion: H.FarLand, H.OldTown, H.Scandinavia, H.Rise+Fall - skip the others). At this level, I estimate Premium can speed up your progress by a factor of 6-8, more if you can get the levels started quickly (join a clan and exploit SAC+FC+request to get to the first market).

d. Post-SuperAscension
You won't get much value right after your first SuperAscension, wait until you reach Far Land for a tangible speed increase.

e. Chain-SuperAscension
Speed increase by at least a factor of x6 at 25%-150% AP (your mileage may vary slightly), going steadily down to x2 once you reach the 250% AP plateau.

f. Late Super Ascensions
Valuable until you have sufficiently advanced artifacts to need only a total of 2 hospital upgrades in your entire run to Hex (those can be skipped with x4 TW that you bring along and recharge at the end of the run)

Beware that once you get used to the speed increase, it's going to be very difficult to adjust back. And by very difficult, I mean painful. If you already know you won't renew your subscription, make sure to slow down during the last week, to help with switching back to a lower gear.

I hope this post was useful, if you have any questions or need a suggestion feel free to ask, I'm quite active in global chat these days.