I have almost the exact same configuration as you, late 2009 macbook plastic, 8 GB ram, samsung 840 SSD.
I did an upgrade for the first time in a decade (I always used to clean install previously) and everything is running smoothly. I was previously using mountain lion 10.8.4, then yesterday upgraded to 10.8.5 and today did maverics 10.9.0. I'd say mavericks is almost exactly like 10.8.5. All the apps were recently updated for both OS's but I didn't have the newest iphoto so I had to bootcamp in order to use photostream. With 10.8.5, I got the newest iphoto so I could photostream. Still I upgraded to mavericks.
All the same annoyances of mountain lion are still here. Snow leopard gestures with 4 fingers don't work for the most part. You can't have right clicking and double tapping, it's one or the other. Battery monitor doesn't show hours left unless you click. But for all the annoyances, you get a stable OS, icloud syncing, and new iLife and iWork apps. Spotlight reindexing took less than 10 minutes (it's nice they give you a timer). All the old drivers I was using with printers and scanners still work, although new drivers are available. Office 2011 works fine and loads at the same speed. Scrolling in Opera seems the same speed, not much lag. Notification center is nice but still has little use for me. I haven't had a chance to try out battery life much, but it's draining a lot faster for me after restarting my mac. Maybe it's not real though. The biggest annoyance in mavericks is that the dock is no longer transparent. It looks really ugly if you have it docked to the side and seems like it wastes space as you can't really put icons behind it since the text is unreadable. I don't like that ibooks came pre-installed, but it's not a big deal. Maps is cool to have around, but you could always use google maps too. Safari is the same as in mountain lion 10.8.5. Dashboard widgets worked fine too.
If upgrading from mountain lion, you actually gain about 4-5 GB disk space. If you update all the apps, that ends up being 2-3 GB because iMovie is a 2 GB download compared to the 300 mb version of the past.
Also, I clean installed mavericks on a stock late 2009 mini (2.26 ghz, 2gb ram, nvidia 9400) and it's unusable. You definitely need 4 or 8 GB ram, but if you had mountain lion, you knew that already. Security wise, there doesn't seem to be anything different. Snow leopard seems to still get updates, so you don't need to upgrade just for security updates.
So if you have 8 gb ram, get Mavericks. It's worth it. Performance is similar to snow leopard and you get icloud. But if you're running on old stock hardware, upgrade your ram first.
Edit: Shut down speed is fast like snow leopard, a big improvement over mountain lion.