I have been using the N900 for almost 2 years. I've upgraded from PR1.1 to PR1.2 and PR1.3 and more recently CSSU. I have installed countless things from extras-testing and extras-devel and have never had any serious problems.
Until one fateful day 2 weeks ago when I started to realise the battery life was getting worse by the day. The battery was draining at about 2-3% an hour on standby. A few days before, I had installed speedpatch and batterypatch. I thought that was the cause, so I promptly removed them. But the issue remained. (I suspect that speedpatch permanently broke the kernel setup somehow, from reading the forums I found others who concur.)
I tried running top and powertop and found the CPU was running at C0 constantly, with 12-20% constant CPU load%. I tried everything I could to find the source of the issue. I un-installed things for testing until I practically un-installed everything I used and it was still the same.
So I was left with no choice but to re-flash the firmware. I had never done it before. I read through the wiki. It was quite confusing at first. I didn't have an Ubuntu box around so I had to use my Windows 7 laptop at work. Unfortunately I had the 64bit version and had to jump through additional hoops to make the flasher recognise the N900. I decided to start from scratch. Backed up all my files and save games and flashed both eMMC and FIASCO images.
Afterwards I restored my contact data from backup but not my settings or anything else. I was advised against it, to ensure the problem does not return from a restored backup. I started to put back all my data & re-install everything I needed. Since I use the N900 as my everyday phone, I decided to install Stable CSSU this time instead of testing.
The fresh image really made the phone much faster and much more responsive than ever before. I've now quite glad I did it and I'm enjoying the phone like it was new. The battery now lasts even longer than the first day I got it. It goes down only 2% in 5 hours on standby.
Increasingly more people are finding they have issues after installing speedpatch and batterypatch. I'd advise against installing these patches.