A friend got the Arc, so I "tested" it for a while.
The phone is incredibly light for its size and thin! The screen is a pleasure to see (certainly compared to my Desire), with its smooth closing in the edges. The build quality feels superb. The speed is fine, so I didn't notice any lags. Arc runs on Android v2.3.2.
Despite its curved lines, the grip is not that good - I was afraid to drop it all the time. The UI feels relatively light compared to Sense, but has less options (can be fixed with another laucher).
I put the Arc (OS v2.3.2), SGS (OS v2.2) and Desire (custom ROM, OS v2.3.3) next to each other to see the winners:
The screen - Arc and SGS, where Arc shows more natural colors.
Build materials - Arc and Deisre. SGS feels plastic.
Performance - Arc and Desire (whereby Desire is "tuned"
). SGS runs on stock Froyo and lags (can be fixed with a lagfix).
RAM - all are the same.
Internal Storage - SGS. Arc comes the second, but I fear if a user starts installing 200+ apps, then it will be the limit. Desire ends is at the end, unless it is "tuned" (which is what I did).
Apps in the phone - who cares? They all come with basic apps, that are mostly changed by the users anyway.
Battery - can't test it yet on Arc. SGS seems to perform better than Desire.
Overall, I see a very good phone made by SE.