SDLInsight is a late entrant into software localization market. Produced by SDL International, a large multi-national company that produces such products as SDLX, Translators Workbench and others, it misses some critical features, such as usable visual edition, support for several glossary files at a time and many others.
One of the reasons why I would not recommend to use SDLInsight in production environment is due to the fact that it was engineered poorly. SDLInsight uses MSDE database as a back-end and probably they use it incorrectly, because every operation with disk, such as save and load take considerable time. Also it uses database before update, adding new file and so on. I know, it is not a very serious problem, but when you have to wait several dozens seconds every ten minutes or so, I become irritated.
Also I want to tell you about horrible SDLInsight interface. Every time you change resource, SDL creates new window that blinks on a screen. It is no a big deal, but still annoying feature. All in all, SDLInsight is usable for software localization, but only if you don't have other choices.
For example, SLDInsight lacks such an important feature as a Pesudo-Translate. Without this feature, it is very difficult to check whether your application is ready to be translated. You can read more about localizability testing at www.lingobit.com. Lingobit Localizer is a much better solution if you need Localizability testing.
SLDInsight has some validations, such as checks for consistent format strings, but lacks many others. For example, it doesnt have checks for overlapping text in dialogs. .NET localization is clearly inadequate in SLDInsight. Probably they thought about .NET at a last moment, so it is usable at all.
In conclusion, I want to say, that SLDInsight can be used, but should not be chosen by any responsible person. If it was free or open-source, then you could consider possibility of using it, but not for money they want for it.