I use the 100mcg/hr patches - With the lowish Oxy tolerance that you have, I'd say that you would be safe with using the 25mcg/hr patch, BUT ONLY AS PRESCRIBED - YOU COULD EASILY OVERDOSE ABUSING THIS PATCH, SO JUST DON'T DO IT. Obviously you know your body better than me though. I have been on Fentanyl over 2 years, and as stated by someone, it seems it is an acquired taste. I hated these things when I was first started on them, but now I'm used to them, and have also found that I my liking for them has grown over time.
Using these things as prescribed, they will NOT MAKE YOU NOD/HAVE EUPHORIA. They are instead very sedating and make you sleepy, they also do wonders for pain. They also take AT LEAST 12HRS to start working the first time you use it, so unless that time period has passed, IT HAS NOT REACHED FULL EFFECTS, SO BE CAREFUL TAKING MORE OPIATES. I found when first starting them, that I felt very groggy and tired on them, wanted to sleep a lot, had a slight headache among other slight symptoms. These wear off after around 2 or 3 days. I'd only suggest Fentanyl as directed, transdermally for pain relief - By abusing these, if you are doing so or plan to do so, you are putting yourself in a position you will later regret - These things jack your tolerance up to the point where no other opiates do very much, and as I'm sure you are aware, this can become a problem. I was also taking 400mg of IR Oxy per day as of 3 months ago along with the patch ( for medical reasons, everything prescribed - long story ), and for me to feel nice and high, I'd need to take 250mg of Oxy at once... That's how much it fucks up your tolerance. If you don't believe me, wait and see!!
If you do indeed abuse these patches and try screw with the time release and put them in your mouth or something - BE WARNED, overdose on Fentanyl is very easy even to an opiate tolerant person, and the sad thing is, you can take too much of it, feel tired, head to bed and end up overdosing in your sleep and not waking up..

Simple as that. This has happened to countless people, so be careful, and don't fuck with this drug, it is NOT WORTH THE RISKS.
Also if you forget to change your patch when its due, withdrawals kick in quite quickly and then you are in for 12hrs of hell while you wait for the new one to start working, it is not fun :S This is due to the short half-life of Fentanyl.. once it starts leaving your body, it leaves quick. This makes withdrawals hugely intense and unbearable, but also makes the period of withdrawal shorter compared with longer lasting opiates.
My advice as far as Fentanyl go: Great for pain relief and helping you sleep. Not so great for recreational purposes due to the side effects listed above such as massive tolerance increase and low euphoria from substantial amounts. I'd recommend something else if you wanna get high, avoid Fentanyl if you're just messing around, you WILL REGRET IT.
** I am not a doctor nor am I any sort of healthcare professional, nor do I claim to be. Anything and everything above is my own personal experience and is not intended as advice. I am not responsible for anyone choosing to abuse this drug. Be careful, Fentanyl is not a joke **