8-speed cassettes tend to have rougher shifting because the differences between each cog (number of teeth) is larger than a 9- or 10-speed cassette, so the chain has a bigger jump to make. It's going to make noise regardless (even the boyfriend's Ultegra makes noise when shifting), but the grindy-crunch noise is really noticeable on low-end shifters and cassettes.
I've never bought a used bike, but: make sure it fits first! Ask how it was stored. (Inside? Outside?) Make sure the frame is sound--no cracks or anything (if you can get an LBS to look over it, so much the better.) Try to look for anything that might be damaged or need replacing. (Tires, bar tape, chain, esp if it was stored outside). Does it come with clipless pedals? (Either get shoes if you don't have them, or a pair of platforms if you're not there yet!) Personally, I wouldn't buy a whole new bike just for the components, especially if you've got a good fit with your Giant. Keep riding, pinpoint exactly what bugs you, then either upgrade piecemeal, or buy a new set once they go on sale in anticipation of the next year's stuff.
All the components on my baby are original, save the pedals and saddle (ouch!). Over the summer, I did the same distances you're doing (and a bit longer, up to 44 miles), and I didn't have a huge problem with the components (except the front derailleur, but it's a problem child). I did miss the ability to shift from the drops, though. One day, I'd like to switch everything to 105 (since that's the highest thing the aluminum Avail comes with), once stuff starts to wear out. Out of curiosity, what year is your Avail? If you don't know, just tell me what color it is. I can figure it out from there.
A new bike...don't get me started. Lighter (just so that I can lift it fully loaded, since I can only barely do that with my current bike), Ultegra or 105 components, nicer wheels. I'm looking at carbon, aluminum, and steel. The biggest thing for me, though, is the fit, since I found that Giant's "women's" geometry really doesn't work for me. It's still too long.





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