I picked one of these machines up a few weeks ago very cheaply (£170 GBP: Apple Refurb £399) and have experimented as the 500GB drive was just too slow for me.
Firstly I used an External SSD as the boot drive and secondly an internal SSD. I was kind of happy with the external solution, it was simple to do and gave me the option of keeping the original drive in place. Ideal to use the external SSD between Macs and keep the internal drive with a fresh copy of Mac OS or just for files. Although the speed was topping out at 300MB/s read and write and it just wasn't as snappy as I'd hoped. Don't get me wrong the performance was faster and Mac OS felt better running on an SSD than HDD, it seems iCloud syncing performed better and I could have lived with the performance - this isn't my primary machine, its a media machine where the OS drags content off another 2 4TB external HDD's. It just felt messy having another external drive and a useless internal HDD.
This weekend I swapped out the internal drive with a Samsung SSD (cloned copy of the external SSD), enabled Trim via Terminal and have been getting read and write above 400 MB/s (Estimated from memory - can update post with actual figures). The machine Geekbenches 3046/5416. I'm happier, it feels like it is now a reasonable machine, completely solid for media, suitable for word processing, web browsing, downloads and maybe even some basic Unity 2D work (Maybe stretching with that last one). The only insurmountable problem now is 4GB RAM!
The external USB SSD was: WD WDBK3E2560PSL-WESN 256 GB My Passport SSD Portable External Solid State Drive, Black/Grey
The internal SSD is: Samsung 250 GB 860 EVO Sata III 64L V NAND Solid State Drive,MZ-76E250B/EU
The SSD upgrade is a must, whether that be internal or external is up to the confidence of the person doing it. I have built PC's before and have experience of Mac Mini's so it took me under 30 minutes from opening the Amazon package to putting it back together. Cloning didn't take much time either as it was External SSD to Internal SSD.