The boxes aren't the greatest but the only real reliability problem with them is a tendency to wear 2nd gear syncro if drivers rush the change. If the box crunches into second then you'll have to replace the syncro and make sure the oil is good and fresh and you should be fine.
The gearchange itself is ok at best, and if it feels really loose or imprecise or binding up you need to check out the linkage - it's pretty simple but the are a couple of bushes and bolts can come loose so worth a look. If you're keen you can improve the linkage but this means making up parts, lathe, weld, etc.
Having said all that my alfasud sprint has 280,000 km on it and the box is fine, nice enough change and no crunching into second. Box has never been apart.