Short answer: not good.  However, with the accomplished Julius Peppers now on the team, the future looks better.  Let's break it down with some stats:

ProFootballFocus writer Khaled Elsayed published a very informative article about pass rush productivity. LINK

They broke down every single pass rushing opportunity for each player and credited 1 point for a sack and 0.75 points for a QB hit or QB pressure to get Pass Rush Points.  Then divide by the number of pass rushing opportunities (minimum 200 rushes, 175 players ranked) and multiply by 100, and voila:  Pass Rush Productivity.

PashRushProdSpreadsheet  
 

Julius Peppers: 9.47 PRP; 8th among 4-3 DEs, 16th among all players.
Adewale Ogunleye: 7.69 PRP; 20th among 4-3 DEs, 37th overall.
Israel Idonije: 7.38 PRP; 3rd among 4-3 DT (but only had 200 pass rush attempts), 47th overall.
Alex Brown: 7.23 (27th ranked DE, 49th overall).
Mark Anderson: 7.04 (29th ranked DE, 53rd overall).
Tommie Harris: 3.78 (26th ranked DT, 131st overall).
Anthony Adams: 3.62 (30th, 137th overall).
Marcus Harrison: 2.41 (43rd ranked DT, 157th overall = turrible).

If you add up all the Bears pass rush stats for the guys with a minimum of 200 pass rush attempts, then you get cumulative numbers of:

2,256 pass rushes,  24 sacks,  40 hits,  98 pressures for a score of 127.5 and a PRP = 5.65 (22nd in NFL)

Minnesota's PRP = 7.39 (3rd)

Green Bay's PRP = 6.24 (15th)

Detroit's PRP = 5.23 (25th)

The Bears were slightly better than the Lions...just great.  /sarc

I did a little math and subtracted out Brown & Ogunleye's numbers and added in Peppers' numbers to get a revised PRP = 5.77.  A very slight improvement (up to 21st in NFL).

What really needs to happen for the Bears' defensive line to get to the QB better is for Tommie to improve...a lot.  His 2nd half of the season was better than the first half.  He needs to carry that momentum into this season.

Next, Idonije needs a lot more snaps; much more than Marcus Harrison, if Izzy ends up playing any DT this season.  He's slimmed down to play DE though, so we don't really know how he'll do.  As a pass rushing DT, Idonije was outstanding.  As a left DE, we don't know how he'll do or how much he'll play.  I'd like to see Izzy play left DE on 1st & 2nd down and then slide over to DT on 3rd downs while MA comes in to rush at left DE.  MA could also play right DE for a few series while Izzy is on the left when Peppers needs a breather or when Peppers plays the left himself.  Also in the mix in the DE rotation is Jarron Gilbert and rookie Corey Wootton.  Will they bring anything to the table?

Finally, our secondary not sucking would help a lot.