The Panasonic systems are actually pretty robust, if you're talking about the KX-TD and the KX-TA systems. I've installed them and used them at several companies where I've consulted in the past, and people have been very pleased with them.

If you were going "upscale" from Panasonic, I would go with Nortel, but their systems tend to be about twice the price, and honestly, I haven't found any difference in long term reliability. Of the 5 or 6 Panasonic KX-TD systems I've been involved in having installed, all are still in use, up to 10 years later, and none have had any major service issues (maybe one or two broken phones here or there).