Not sure about flaws, but I've parked next to one or two and their owners love them. Quite a voluminous boat for a 28 footer. I sailed a Mirage 28 once which is quite similar (same designer) and might be a good alternative.
No major flaws. Woodwork was a bit on the light side, I think, some fittings too but generally a sound coastal cruiser. They and others were hit for six by the French invasion of the late 70s, early 80s. Beware the many kit-built versions. At current prices a well-presented one would be good value.
I know owners of the 750, 850 and 1050. None of them have a bad word, and each has owned it for ages.
Not cutting edge performance but pretty solid and used a lot.
I can confirm that a factory finished 850 is an excellent boat!
As regards performance, well the 850 came in two versions, Bilge Keel and Fin keel. David Feltham has said on more than one occassion that even today, the only thing he would actually change about the 850 is putting about another half a ton of ballast in the keel(s) to stiffen her up a bit! They do tend to be a bit on the tender side, and certainly benefit from an early reef, BUT having said that, they do make a superb family cruiser that will take you anywhere you want to go around our coasts and across the channel.
Worth your while having a look at the Cobra Seawolf Association web site (as per the link in Savageseadogs post above!).
Other than some home built boats of indifferent quality, the only other thing you have to look out for are those boats that were ex-charter. The Cobra 850 was one of the very first boats that was introduced to the Meditteranean charter market, and there are many 850's in varying states of tiredness in the Greek Islands.
As with all secondhand boats...if you are going to buy, don't trust your own judgement, let a surveyor give an opinion!
We had a share in an 850 for 8 years .She was bulit in 1979 and had been a charter boat .She was very solid and sound and still looks a fine boat for the money .She has been well maintained and over the life of these boats I would imagine most gear will have been replaced a some time or other .She sailed reasonably well to say her sails were well past their best .I cant think of any particular vices and I would certainly have another .As with any boat you need to have a very good look round and allow for any potential exspense .Iv seen tired charter ones bought back to life and going very strong and they were bought at a very low price .Our old share boat is still going strong ,is used throughout the summer and shows no signs of being ready for the scapyard .
I'd noted your name, and wondered if that was a link to the Cobra 850 - 'Ostra', which I delivered to Greece in 1978 or 79 for Crawford Perry Travel. Then noted you have a Moody . . .
Biggest shortcoming of those old Cobras was their very agricultural (literally) old single cylinder YSM and YSG Yanmar engines, which were a little light on power. Also, there was a fault in many which permitted the mast foot to drop a little - most of those have settled though, with slightly shorter shrouds!
The slight tenderness also means there's rather a lot of weather helm in gusty conditions.
But, space for your money, and robust build? Excellent. By now most of those on the market will have replacemnt engines, replacement rigging, fourth of fifth suit of sails, dual battery installations fitted etc etc.
I Had an 850 for about 8 years. Loved every minute of it and regret selling it.
DONT get one with the 'original' heavy BMC engine as it seriously effects the weight and trim of the boat. A modern lighter engine seriously improves every aspect of performance.
Get a good downwind sail, (the bigger the better!)
Mine was a bilge keeler and it did slam a bit, but the creek crawling abilities made up for this. Would have a bilge keeler again.
The hull was pretty strong and I never felt the boat would let me down. I applied a copper antifouling to mine and the clear gelcoat below the waterline was in perfect condition. I am told that clear gelcoat is less prone to osmosis (but this may be BS!)
My rig and sails had little in the way of adjustment and in hindsight a few opportunities to tweak would have also helped performance.
As has been said, many were home completed, to various standards and designs. Find a good one and I am certain that you would not be disapointed.