I've used a number of these types of storage solutions. First of all, I would never trust anything LaCie (to be fair, I've never used any of their four-drive boxes, but I've had a huge collection of failures on their dual-drive units (they sell a lot of very attractively priced mid-high capacity dual-drive systems, which I was using (and installing for others) a lot of for a while until I started to notice their elevated failure rate). A colleague has both an Apple XServe RAID (in for repair right now, but ran flawlessly for four years, and I'm pretty sure the repair won't result in data loss) and a newer Promise RAID (the unit Apple now sells), which is flawless after about a year and a half - never lost a drive on either one, and the Promise is RAID 6 (can tolerate double drive failure) PLUS hot spare (will replace a down drive automatically). The Promise is big, noisy and expensive, BUT it seems as close to foolproof as you can get - you'd have to be pretty blind not to notice a TRIPLE drive failure, and the drive you change in any failure always comes in as the hot spare, so there's no risk of detonating your data by pulling the wrong drive during a rebuild (you let the rebuild happen onto the hot spare BEFORE replacing the drive). The 16 bay Promise costs close to 15 grand fully populated, AND wants to live in the basement (noise), but it's as safe as you can get short of an offsite backup (and have you priced a T3 line lately?). One big advantage of 16 bays is that the percentage loss to parity is very low - RAID 5 always takes one drive for parity, and RAID 6 takes two, no matter how many drives there are in total, so the Promise running RAID 6 plus a hot spare actually loses less capacity than a 4-bay system running a barely adequate RAID 5!
I'm using an 8-bay EZQuest myself right now, and it works well ($3500 for 8 TB - $5000 for 16 TB). Unlike the REALLY expensive systems, the intelligence on this sucker is in the PCI-E controller board, not the RAID case. I bet a homemade version of this could be put together somewhat cheaper - the RocketRAID card they use is about $400, then any 8-bay case should work - unless the 4 eSATA to single InfiniBand connector they use is very expensive. The case connects to the card using 2 InfiniBand connectors, each of which is carrying 4 eSATA channels (it's just combining the signals, not translating). My next one will be a similar concept, but probably homemade. I'll also use RAID 6 next time - this one is RAID 5...
-Dan