There are some nice areas out there in Waltham, but doing research is very prudent -- including if there are any pending lot exceptions about to be built right near your property of interest. You can check the building permits for properties in the vicinity, and see if there is pending construction nearby. Another tip may be seeing if the property you are considering is already less than the zoning minimum required for the district. If it is, you may be finding yourself soon in a more crowded neighborhood than you realized. The zoning maps and zoning requirements can be seen on the city website, and compared with other city and towns in Massachusetts that freely provide that information online as well. The lot sizes of properties on the street can be seen on the appraisal listings also available on the Waltham website, as well as the date of the development of the property. This can give some sense of stability in a neighborhood.
It's a nice city in some respects, but overdevelopment is definitely a concern in this city. Future Waltham home values may decrease due to this continued crowding of certain neighborhoods. For myself, buying a home in the Waltham Highlands has taught me some unfortunate lessons, especially about zoning practices in this city. My hope is that others choose their location wisely, and avoid the problems for their new home choices.
Here is everything actively on the market in Waltham with a map:
Essentially, for single families, south of Main St is less desirable and the neighborhood really looks more 'messy' than the north. However, the North doesn't have the quick access to the famous Moody St (restaurants) and not as easy to rely on public transport as the South.
Usually, the more urban, the more mixing... so the messy-feel of the south is a results of mixing multi-unit apartments, high end condos, projects, etc. all in the same vicinity. Whereas the north part of town is more uniformly single families and looks more safe.