(Parts of this are from a previous post I made to a similar question last year.)
A couple years ago I evaluated a Cisco 6500, a Black Diamond 6800, a Nortel 8600, an Allied Telesyn SwitchBlade 4000, an HP ProCurve 9300, and a Foundry FastIron 400 side by side.
A little background: Once upon a time we used IBM ATM equipment in our core and migrated to Nortel's 8600 platforms when they first came out. We've been very happy with them but when the 3 year leases started to expire I wanted to make sure we knew what our options were.
Here are a few (random) thoughts:
First we chose Nortel again, partially because of aggressive pricing offered and partially to avoid retraining staff. Good experiences with stability, the local support team, and features like SMLT also helped. I didn't have any experience with the other platforms, except for a familiarity with Cisco's ISO because our WAN routers are Cisco models.
We are a 24x7 operation so uptime is critical but we don't do anything too exotic, most all of the platforms passed all of my tests. In general I ignored the vendor-supplied performance numbers since they all use Enron-style math when reporting speeds and performance.
I thought the BlackDiamond had the best interface in terms of usability, even better than our Passport 8600 for a new user. On the flip side I thought the Catalyst 6500 had the worst interface for a new user, I just don't think an interface designed for a 4 port router works on a 300 port core switch. I also was annoyed that because the 6500 has both the CatOS and IOS operating systems and since they have different features available for each it seemed like (at the time) we could either have a common IOS interface with a reduced feature set or a better feature set with a proprietary interface.
I did like the wide variety of options for the 6500, the NAM blade and netFlow features were quite cool. Nortel says they'll support IPFix in 4.1 but it seems that they're loading an awful lot onto the plate for that release...
I haven't opened very many trouble tickets but I'd classify my luck with Nortel support as 'good but somewhat mixed'. As with a lot of support situations it seems like getting the right person can make all the difference... I've called Cisco a few times for router issues and had mixed results there as well. Once upon a time I worked on a model of DEC router and had a really complicated configuration, I called for support quite a bit and when I did the first thing I'd say was: "it'd save a lot of time if you just transferred me to Dave right now." They usually did and Dave always came through for me... he probably doesn't know it but I've toasted him many times over the years.