I also have the Seidio 3500 and have had mixed results. Im not going to knock the battery, this is more for any possible help.
I received my first 3500 and for about a month or so I was beyond thrilled. I would get about 3 days between charges, and that was with me using it generously. After the month, the battery stopped holding a charge, and would literally nosedive upon disconnecting it from the charger. I went from 3 days to - no joke - about 10 minutes. I returned the battery and received a replacement.
The second battery was actually too short and didnt fit in my phone properly, and so it was difficult to charge, and my phone would just shut off because it would lose contact with the battery connectors. This went on for a week, and then I returned it and got a replacement.
The exact same thing happened with the third battery that happened with my first one. Worked awesomely for a month or so, and then it just tanked again. I returned the battery, and received a replacement.
The fourth battery was returned the day after I received it because it wouldnt charge at all. Just refused. I tried multiple times, and nothing. Back it went.
Im currently on my fifth battery, bought it back in July, and again, it was awesome. Was getting 3 days between charges, again with liberal use, not having to worry about turning anything off (although I did download Juice Defender). Then about a week ago I noticed that I was losing about 30% of my charge in about the first hour after it was unplugged from the charger. I started watching my usage, but it didnt matter, it would continue to go down steadily. I still get about a full days worth of use out of it, but thats with moderate use, gps off, etc.
Each time Ive had this problem, Ive contacted Seidio and they told me to return it and get a new one. But as I told them, thats not really solving the core problem.
Has this happened to anyone else? Any/all suggestions are welcome.