Just wondering if there's a rule-of-thumb on replacing vs. repairing central A/C units?

My home in MA is 8 years old, and has a (rather cheesy looking) "Concord" system installed when it was built. Last night it
stopped cooling - the inside & outside fans run, but no compressor sound, warm air, and the lights dim every 30 seconds or so
like the thermal overload is cycling. I shut it down last night and tried again in the AM with same symptoms.

So I guess the real question is, if it comes down to replacing outside unit + inside coil vs. just replacing the compressor
(do people even do this any more?), what's the best choice? I assume the newer units are more efficient, but how long would
it take to recoup an additional $1000 or $1500 cost?

Eric