If its a domain name your planning to hang onto go ahead.
If you're buying to resell its best to avoid them. Last time I looked there was no push mechanism between Yahoo accounts. I dont know if this has changed in more recent times.
I dont know how difficult it is to transfer out to another registrar from Yahoo, as they are bound by the ICAAN rules, but I seem to recall that when they had specials running they had no transfer provisos for long periods of time in their terms and conditions.
Read the terms and conditions currently being used very carefully is the best advice I can offer.