I am really starting to *HATE* this feature because of its lack of usability... First off, I have NEVER yet been able to get a trigger off of any reminder that I've set using the location. If I enable location as well as time as triggers for the reminders, the reminders will only trigger when the time has come (i.e. when the time set in the reminder has passed). A location-only reminder has NEVER YET WORKED for me.
I have Location Services turned on and ALLOWED the use of these for the Reminders App, I've set up iCloud and enabled reminders to be synched there, I have disabled synching reminders to any other account (so that only iCloud will receive them) and when setting up a reminder for 'leaving' I checked that the 'Current Location' was indeed my current location. At least on this final point I can tell you that the 'current location' was always as accurate as could be, even when within a building.
Despite all of these precautions, not a single one of my location-based reminders have worked to date. I have tried it five times now and decided I shall NEVER be using the Reminders app again (at least until such a time as when it is fixed). I cannot and will not believe that the problem lies with my GPS accuracy, because, as I mentioned before, even the Reminders app itself displayed the correct location when using it for a 'Leaving' reminder. Secondly, I have two Apps available that can do the exact same thing in the situations for which I use it (reminding me to report myself when arriving at the railway station), and they DO work.
As you might be able to guess by now, I am really disappointed in the way this feature works (or rather, doesn't work). If I use my Satnav application (Navigon Select) it will always tell me I have arrived EXACTLY at the right moment, yet I still don't get the reminder to trigger.
The only possible cause as to why this can happen is that Google Maps doesn't pin-point the house number. So, if you set a reminder to remind you at 1, Baker's Street, London, it will actually display your location as being 1 - 7, Baker's Street, London. This means that a radius exists, and that is where this fails. The address you've set as your location reads 1, Baker's Street, London, and therefore doesn't match with your current location.
(Please note that any correspondence of street names used in this post to actually existing street names is entirely coincidential).
I live in the Netherlands and I have an App installed that 'wakes me up' when I'm arriving at a set railway station. I can set it to trigger 'exactly at' the location, or otherwise when within 20, 10 or 5 kilometres away from there (in other words, when I am within the desired radius). No matter which of these settings I use, all of them do work, including the 'spot-on' one. So, apart from the hypothesis I pointed out above, I cannot seem to find any reason as to why this wretched Reminders App doesn't want to try and do the same just for once. I feel Apple shouldn't have tried and come up with such a feature at least until such a time as when they were convinced it works in 99% of cases. It seems that, currently, it's coming closer to a 9% score. Rather embarrassing, if you ask me. So, instead of using this excuse for a Reminders App to remind me (or not), I've gone back to using apps that do quite much better at it. And as an added bonus, these also work on my ye olde iPhone 3GS, and didn't even need iOS 5 (although that is now installed on all my iDevices).
To sum up, I think this App (the Reminders app) is just about as useful as an empty teabag when it comes to reminding you. I think it could have been potential, seeing as it does know (and display) my current location correctly. Shame on you, Apple...