We loved our place in San Telmo. HUGE open plan, split level apartment with balcony, kitchen, living area, great queen bed with excellent sheets.
Location is excellent, a short walk to Line C Indepencia subway station. You are also close to all the great local restaurants in San Telmo. Try Saltanapoli just around the corner which does cheap, wonderfully authentic Argentinian food from Salta. There are also small local groceries, bakeries etc, everything you need.
Another tip: How to get laundry done cheap! Walk out of Piedras, turn right onto Estados Unidos, walk a couple of blocks and you'll find a laundry on your right. You can do your own laundry in the machines, but why bother when they will do your whole bag for a mere 48 pesos! They don't speak much English though so write down your request in Spanish before you go (Google Translate or ask the front desk guys).
Some people will tell you the area is dangerous but we never felt unsafe at all, even walking around at night. Obviously crime happens in BA so use your wits and you'll be OK.
Our only complaints would be that the sofa was pretty uncomfortable for sitting because it was a cheap sofa-bed. Also, most of the guys on the front desk speak excellent English but you'll find occasionally there is someone who doesn't speak alot of English. You'll get by though as they are friendly and helpful.
I would definitely stay again, Piedras is excellent value for money!