So I tried experience keys (rant) (Patreon)
Content
Here's a comparison for force-attachments:
Cost
Experience Keys: Premium Account
RLV: Free
This is the first major advantage RLV has. If you get banned or quit SL, RLV will continue working regardless. If you use an experience, everything breaks and there's nothing the consumer can really do about it.
Region
Experience Keys: Land-locked
RLV: Grid-wide
This was the deal-breaker for me. Even if you get a premium account, you have to own land (which is ludicrously expensive) and then your game would only work on that land. Imagine if game of thongs ONLY worked on Quiddity. That would severely handicap the game. There is a grid-wide key you can obtain, but there's no information about it. I've sent a support ticket to LL and will update once I get a response.
Availability
Experience Keys: All viewers
RLV: Third party
This is one of the places where experience keys wins. Though not by far considering that third party viewers are far more used than the official one.
Open Source
Experience Keys: Limited support
RLV: Full support
This is another dealbreaker. You need to be granted permissions to compile scripts for the experience. This makes the experience permissions more secure than using RLVs @accepermissions for forced-attachments. However, it also means that only a select few will be able to work on the project, which essentially DESTROYS the idea of open sourcing and allowing anyone to mod the code. I know you could theoretically give experience permissions to everyone in the group, but that would make the system even LESS secure than RLV as you'd be able to force-attach temp attachments even when you're not wearing the HUD.
Security
Experience Keys: Good when closed source, nonexistent when open source.
RLV: Decent
An issue with temp attachments is that with RLV you might receive some with exploits, which then stay on your avatar and can do things like track you without your knowledge. With experience keys you can blacklist certain experiences, but experience keys can be exploited as well unless you make the item closed source and the HUD no mod.
Functionality
Experience Keys: Flawless.
RLV: Buggy
This is the main reason why I wanted to use experience keys in the first place. Some times @acceptpermissions will be ignored by the viewer, and you will still get attach permission request popups.
In conclusion I'd say the way they implemented experience keys was a massive waste of time. I can only hope that Linden Labs go back and revise it completely (make it pay per experience like Groups), or just leave it dead in the water where it's currently floating.
Update1:
I've filed some JIRAs to hopefully help improve security without removing the RLV attachment feature. Feel free to go and comment!
Firestorm (improve security): http://jira.phoenixviewer.com/browse/FIRE-19492
SL (temp attachment lists): https://jira.secondlife.com/browse/BUG-20047