Cheap (older) SSDs can be used when u read alot, but i would avoid them if you do alot of writing too. If you only use it for the system drive and you don't reinstall/change stuff all that often, cheap SSDs will give you the benefits of quick booting and quick application launch, combined with the physical benefits of SSD: reliability, virtually no power consumption and virtually no heat/vibrations/sounds.
So old SSDs can still be usable. Note that the actual speed of an SSD is determined by its controller; not the flash memory itself. Thus newer controllers can provide many times the performance of older SSDs especially at the areas of writing and more complicated I/O patterns. Also, older SSDs would lack TRIM; but usually because these older SSDs lack remapping writes this is no concern. These kind of SSDs will always be slow for writing; TRIM wouldnt have any effect on them.