Ive been told that the thompson is a very hardy gun. People argue which one is better the TM AK or the TM Thompson. Why? Because they both have alot of metal on these guns. Back in the day most AEGs didnt come with alot of metal right from the factory, if you wanted the metal feel then those were expensive additions. Now a days most AEGs have more metal as their counterparts would have. Still, the thompson and AK seem to just feel heavier and the barrels feel like solid metal. Then I've heard people say that the thompson and Ak have been the most reliable gun they ever had. So the Thompson in short has a good reputation.
Secondly a friend of mine bought a Thompson. It feels nice and heavy. The plastic wood stock and grip replicas are done very nicely and look like real wood, I couldn't tell it was plastic untill I held it. You will have no barrel wobble problems with a thompson. I've never heard of it, its a rock solid gun.
Thirdly, upgrading. The downside of the thompson is that it uses a ver 6 gearbox and parts for the internals are very hard to non-existant to find. For example if I were going to upgrade an AEG to about 380-390 FPS then I would want to buy a silent piston head set to ease the extra wear caused by the stiffer spring. But I have not yet been able to find anything like that for the ver 6 box. I suppose you could throw in an M120 spring in but you will wear your gun out faster. You still can find stock parts for the gun but those can be hard to find at times.
If you get one I would recommend leaving it stock and you will love it! You can put in a heavier spring but you may end up with frequent repairs, but its all up to you. Plus with a heavier spring you may need a 9.6v battery and I dont think the stock has enough room to hold that extra cell. Oh and one more thing, the stick mags for the Thompson are one of the most expensive mags out there ranging from $45-$55 per mag. And there is only about a $5 difference between hi-caps and standards. So just be aware of this added expense when looking for one.