Well I replaced a SP3 for a SL (i5/8GB) and so far is been working perfectly fine, the screen is absolutely stunning compared with SP3 even the aspect ratio is more friendly then before.
I confess that for me the constant concern is the Keyboard reliability, I even bought the i5/256/8gb for cost effectiveness in case of this keyboard become so damage that I have to buy a new laptop, sure that I have 2Y warranty although I'm not sure how's that gonna work with MSFT support (so far they failed on me with SP4 type cover that working 8h/5d got serious usage damage and start to fall apart on the fabric).
I've been following SL on reddit and is almost inexistent for problems, but keep in mind, that when if fails on you, you better have some cloud / backups working or else you'll loose everything because you have no moving parts.