I've always checked syndicate agreements against common sense, but never used a lawyer. 8 syndicates into my flying career, and a few headaches - I've never yet actually regretted not taking legal advice.
The three things I'd consider most important to check are:-
(1) What sort of people are the other owners
(2) What condition is the aeroplane really in, checking the aeroplane and the maintenance records thoroughly
(3) What's the availability like.
I do mentally write off the cost of any new share over 3 years - then if I can sell it one day and get money back, I consider that a bonus. The last share cost me £3,500, the aeroplane, a Stinson 108, did turn into something of a money pit, and we eventually sold the aeroplane - from what I got about £1200 back. In perspective, my "through life" spend on that aeroplane works out about £160/hr, on something that you couldn't possibly rent, but the nearest equivalent would cost abour £200/hr to rent. But with that, I routinely took the aeroplane away for days or even a week or so at a time - and that's just completely impossible renting.
My current touring share, an AA5a, I've owned 1/20th of for 4 years - my through life spend on that works out about £115/hr, compared to £150/hr to rent an identical aeroplane from home airfield. Drop that to £100/hr if I sold it tomorrow for the £1200 that the last share changed hands for.
My microlight shares - current (a Mainair Blade) and previous (a Thruster TST) both worked out around £45/hr through life.
G