I have Kelly Take-Offs on my rain/beater bike and they work very well. They aren't as quite as convenient as true STI/Ergo brake/shifter (aka brifters) but they are the next best thing and far more convenient than either downtube or barend shifters and yes, I've used both of these too.

The Take-Offs are back in production so they are neither rare nor expensive. The brackets themselves are $50 a pair and the complete installation kit (brackets, cables, housing, downtube housing stops, etc.) is $80. Look here: http://kellybike.com/2nd_xtra_takeoff.html

Above that the only cost is a pair of downtube shifters and you already have these. The nice thing about the Take-Offs is they never need to be replaced. If you want to upgrade from 7-speed to say 10-speed, as far as the shifters go all you need is a pair of 10-speed downtube shifters.

They are a very inexpensive way to upgrade an older bike to brake lever mounted shifters at relatively low cost. New brifters are very expensive and used ones are always a crap shoot.