The Streak is definitely a unique device with niche users. I really like mine, but I've had a Streak for ~8 months and it was first released more than a year ago (June 2010). I would *not* suggest it for someone who's buying it today, with one of the reasons is the somewhat outdated hardware in it.
My biggest complaint is the terrible quality control in the production line. It's true that with any product you'll get some lemons, but I found that the Streak is particularly horrible in this case. The early batches were mostly bad, LCDs (underneath the gorilla glass) breaking on its own, random overheating, slowness and crashes, defective internal SD cards, etc. The later batches are much better.
I'm going as far as saying that with the Streak it's pretty much a gamble which kind of device you're going to get, a perfect one (if you're very lucky), or one with something wrong with it. Something that you won't know what it is until it's acting up, and it can be absolutely anything. Screen, battery, internal SD card, you name it. Of course, Dell will replace it for free very quickly when these happen (I got my replacements ~3 work days from when I called in), but it's still a pain to have to do that.
The Streak is definitely not for everyone, but those who like it usually really really love it. However they have to go through the 'trial' first. The lucky ones got their first and happy with it, some will have to go through replacements after replacements until they got their good one (I've heard of someone on their 6th). I'm on my third one now. It's been about four months, it doesn't seem to be showing any strange behavior, so I'm really hoping this is going to be my last one
If you are planning on getting it with a subsidy for a two-year contract with AT&T: forget about it. You will regret it and curse yourself for the next two years because you're stuck with it.
If you are buying it outright and can get a significant discount, older hardware don't bother you much, you're not shy of rooting/flashing custom ROMs, and (most importantly) screen size is very important to you, then it's worth to think about. There's nothing else out there with 5" screen, and I'm guessing there'll never be another one.
Edit: I forgot that you asked about the on-screen keyboard. It's basically nothing to write about. I believe the Streak is now shipping with stock keyboard and also Swype. Some people will find that these are enough, but just like all Android devices out there, most will want to try something else from the market and find what they like. I've tried several of them and am really liking Swiftkey so I'm using that now.