The TI555 (and 545) is a very capable platform. When combined with a CTI2573 ethernet card (latest version is 100mb/s capable) this PLC can be used on an ethernet network. I currently support close to 20 PLC's in this type of installation. I use Soft-shop (hated TI_Soft but that's a personal preference thing I guess) and can connect to any of the systems via our corporate WAN. One of the sites uses a combination of (1)555 and (13)545 processors. The 555 has 6 remote base controllers (remote IO) and I also used 2 S7 IO racks connected to the Profibus port for remote IO (didn't want to shut down to add another RBC). The CTI ethernet driver is used to connect all of the PLC's to a Wonderware application. We also transfer data from/to the 555 to the other PLC's though the more critical info is hardwired. We also install TI545/555 PLC's as data concentrators in systems that make use of DCS technology. Most of the time, using a serial port to gather data to a DCS from a 3rd part device is not paricularly cost effective to do if there is more than one device. We use a CTI2573-MOD card to collect the date from 3rd party devices on one port (RS485 multidrop) and set the other port up as a MODBUS slave and connect it to the DCS. We also install a CTI2572 ethernet card so that we can add points to the poll list without having to go to the site.
Our largest 555 application has 65 PID control loops, 145 analog alarm blocks, around 1,000 rungs of ladder logic, somewhere in the neighborhood of 500 - 600 IO (analog and discrete), and numerous serial links to other devices. We also have 20 special function programs that perform some of the functions that are required for some of our advanced control loops, perform AGA7 gas flow calculations, perform scaling and unscaling, perform high and low select functions, and many others. Special Function is probably one of the handiest options that the 505 makes available.
I've used the S7 and it's ethernet card. It wasn't as robust as the CTI ethernet card in the 505 series PLC is. For instance, I've never seen the CTI card fail to accept a connection. Even with 13 other PLC's, my programming software, some trending software that I occasionally use, and up to (6) large, independant Wonderware apps (8,000 tag) banging on the ethernet port the CTI card never fails to deliver data. The WW apps are polling the driver (independant driver on each node) at 1,000ms intervals. The S7, on the other hand, did not seem to like having 3 or 4 nodes connected to it at the same time.
I love the S7. It's a great box to program. However, if given the choice, I will install a 505 series PLC every time.