I love my PB12+SSD
I bought a 64GB CFD branded SSD for my PB12 in 2011 and have been using it daily since.
http://kakaku.com/item/K0000291302/
About $US90 at current prices... About 120 when I bought it 2 years ago.
Installation was a bit tedious but simple nevertheless.
Performancewise... It turned my beloved but sluggish PB into a usable beast.
Notable improvements include:
Boot up. Application startup. Spotlight and Quicksilver search.
Especially, Office 2008 becomes usable whereas I had to stick with 2004 for usability before.
The best thing is multiple apps.
I typically run Safari Scrivener Evernote Omnifocus Busycal Mail.app Terminal Activity Monitor and a combination of the Java stalwarts Tufts' VUE, CMAPTOOLS and Freeplane all on Leopard 10.5.8. Usually with all but one or two simultaneously.
Background tools include Quicksilver, Dropbox, SizeUp window manager and DefaultFolderX.
With an SSD, i found this setup eminently usable, if a little slow. though the real drag being Dropbox. Closing Dropbox for the duration of scrivener use made my laptop completely usable if not buttery smooth.
The poor little thing even manages a wee bit of excel without closing down scrivener et. al. Because the SSD REALLY helps with swapfile virtual memory.
However, I since moved my ssd out of my PB12 and into an old Mac mini on iTunes server duty and put an old 100GB 7200 back in.
Crickey, one forgets just how slow the old dog is with spinning rust instead of silicon...
The thing really is painful. Without the SSD, Dropbox brings it to its knees. I have to disable it to get any work done. Also, spotlight is slow as a really slow thing on a hot day. forget using Scrivener and Excel with Evernote syncing at the same time. Once memory runs out, the thing just beachballs every time you press a button or click an icon for the first time.
Oh, I forgot to mention, my PowerBook flew off a table, through the air about six feet and onto solid concrete during use when a cockroach hit me in the face as I worked at an outdoor café last summer. Despite some serious dents, it continued to work without pause. I'm not sure an HDD would have been as lucky.
battery life? Under normal, single app use, I found some difference, a battery charge lasting easily 4 hours or so. But remember one can also get stuff done quicker without the HDD slowing you down, so it is more efficient in that respect.
All in all... The SSD gave me 2 more years of usability out of my PB. In fact, as I am debating a new Air yet see no need to upgrade any of my software, I might instead plunk for another 64GB SSD and see if I can't get another year or two out of it!