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I agree with Tony and Gary, I used to own a Synta 150mm f/8 scope and it was not great for observing planets (not so bad on Saturn, worst on Mars and Jupiter). The colour error was huge on bright objects (it exists with faint objects too but you don't see it) and the scope was quite big (~4ft long and ~10kg) and awkward to handle, in part because it is very nose heavy. The websites you mention show the scope being held at about its midpoint, this will only work if you hang extra weight off the rear of the scope. In reality the balance point for the f/8 version is ~1/3rd of the way from the objective to the focuser meaning that you need a tall tripod or pier to mount it on or you are going to be on your knees when observing near the zenith. Speaking of mounts, IMHO you will want an EQ6 or stronger mount to adequately cope with the big moment arm of this scope. All up, I much preferred using the slightly smaller 120mm version of this scope which weighs just over half as much as its bigger sibling and balances close to the midpoint of the objective and focuser. The reasonably priced 120-130mm ED scopes available today are much better again than a 120mm achromat. If you can give up on the idea of a refractor (but still want some glass up the front of the scope) the IM 715D mentioned above should beat any 5" or less APO on planets and give a 6" APO a good run for its money.
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